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1990 Wrangler 2.5 TBI surges but only after a few miles


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Before I shove this off a cliff... I need help please.  I bought a cheep 90 wrangler 2.5 tbi 5 speed with a crate engine in it.  It will run perfectly fine for the first few miles (I've driven up to about 10) before it starts losing power, bucks and backfires.  I can shut it off for a bit and then drive again just fine for a short bit then it acts up again.  It seems to idle high as well (1100ish) and sounds cammed when first warming up.  I have some experience working on cars but not a ton.  So far I have:

 

Fuel related:

1.  Dropped the tank to check and see if the soft lines are pinched

2. Checked the tank (plastic) for debris and ran a magnet in to get a small amount of rust out

3. Replaced the gas cap, fuel filter, and fuel pump relay

4. Blew through the lines to make sure they are not clogged 

5. insulated fuel lines near the exhaust

6. installed new injector 

but I can NOT check fuel pressure at the test valve because its rounded out

 

Spark related:

1.  Changed the plugs, wires, cap, coil, and rotor button

2. Changed the ignition control module

 

Sensors replaced:

1.  Crank postion sensor

2. Intake coolant sensor

3. MAP sensor

4. Intake temp sensor

5. O2 sensor

6. CHECKED the TPS

 

I also replaced the thermostat and the coolant is always in check.  A compression test (cold and dry) showed 150 to 157 so that seems fine.

 

This is my first jeep and honestly want to love it but I'm frustrated.  

 

 

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Frustrated...I understand your sentiments.  Let's walk through this a bit and see whether a solution can be found.  See my highlights in red...Moses

On 1/11/2022 at 5:17 PM, Frustrated Jeep Owner said:

Before I shove this off a cliff... I need help please.  I bought a cheep 90 wrangler 2.5 tbi 5 speed with a crate engine in it.  It will run perfectly fine for the first few miles (I've driven up to about 10) before it starts losing power, bucks and backfires.  I can shut it off for a bit and then drive again just fine for a short bit then it acts up again.  It seems to idle high as well (1100ish) and sounds cammed when first warming up.  I have some experience working on cars but not a ton.  So far I have:

 

Fuel related:

1.  Dropped the tank to check and see if the soft lines are pinched

2. Checked the tank (plastic) for debris and ran a magnet in to get a small amount of rust out

3. Replaced the gas cap, fuel filter, and fuel pump relay

4. Blew through the lines to make sure they are not clogged 

5. insulated fuel lines near the exhaust

6. installed new injector 

but I can NOT check fuel pressure at the test valve because its rounded out...Yes, you can check the pressure.  Make a test Tee to fit at the inlet line to the TBI unit.  Run a fuel pressure test gauge off the Tee.  Since the pressure regulator should hold pressure at 14-15 PSI under normal conditions, the inlet pressure near the TBI unit should indicate that pressure...Consider this:  The regulator allows fuel pressure above 14-15 PSI to return back to the tank.  This drops the fuel pump pressure reading at the inlet to the TBI unit and reflects the pressure regulator setting...In light of this, make certain that the return line to the fuel tank is unrestricted and that the rollover valve(s) and vent lines to the fuel filler are not restricted.  Lines must be in correct order, including the EVAP system.

Check the EVAP solenoid function and also the EGR function.  Each can be tested with a simple vacuum pump.  (Search under "EGR" and "solenoid" at our forums, we have been over this on 2.5L engines several times.)  Like other parts, the EGR can stick when hot, especially if carbon has built up.  Check the EGR function cold and hot without burning your fingers!

 

Spark related:

1.  Changed the plugs, wires, cap, coil, and rotor button—Not a bad call on these parts, the symptom you describe can include a hot, defect coil, the ignition module or even the ECU—especially at higher mileage.  The ECU can also fault when warmed up or hot.  I see that you replaced the ECU as well.  You also replaced the ignition module, reasonable enough, though these parts can be tested before arbitrarily replacing them.  I will note that there's nothing wasted when replacing high mileage electrical or electronic parts.  If they haven't failed yet, they will.  This is just not the best troubleshooting technique.

A factory service manual is a must, especially for any 2.5L model with TBI.  I use a 1989 Mopar factory print manual for 1987-90 YJ Wrangler and 1984-90 XJ Cherokee 2.5L TBI work or diagnostics.  The manual is packed with service data for testing each of the fuel and spark management system components.  A manual is ultimately far less costly than a parts replacement approach.  You can find a CD version at eBay, and a PDF/CD reprint with bookmarks is much easier to navigate and handle than the original print manuals.

2. Changed the ignition control module

 

Sensors replaced:

1.  Crank position sensor

2. Intake coolant sensor

3. MAP sensor

4. Intake temp sensor

5. O2 sensor

6. CHECKED the TPS—The TPS can test good cold then fault when warmed up.  Test TPS values with the engine cold then hot, ideally when it acts up.  The TPS (throttle position sensor) is like a potentiometer or rheostat.  Voltage increases and decreases according to the movement and position of the throttle valve.  It is not uncommon for the TPS to fault or fail by losing calibration as it heats up.  This device cycles incessantly over time and wears accordingly, aggravated by engine heat and vibration.  We bought our '99 XJ Cherokee 4.0L at 94K miles with all original parts.  By 140K, the TPS was done and needed replacement.

One thing I would add to the TPS possibility is electrical resistance.  This can be anything from a defective alternator, bad grounds (notoriously the engine to firewall ground and others on your YJ model) and even a defective battery or poor battery terminal connections.  Think in terms of junctions and other sources of resistance.  If you have in inexpensive non-contact (infrared) thermometer, do some underhood heat checks at the alternator, manifolds, intake system and the grounds.  Run a simple lamp load test on the ground circuit(s) to confirm wire and terminal integrity.  The YJ Wrangler firewall ground is always suspect, especially when hot.  D.C. 12-volt wiring and connections are as important on the ground circuits as the hot or positive side.

I also replaced the thermostat and the coolant is always in check.  A compression test (cold and dry) showed 150 to 157 so that seems fine.—Compression is very acceptable and, better yet, well within the 10% limit between high and low readings.  A leak down test would be more precise, but for purposes of tuning and driving, you're well off.  If there is little or no oil consumption, no knocks and good oil pressure, you have a lot to work with here.  Manifold vacuum at idle should be normal, check it to be sure.  Make sure the intake manifold is sealing well.  Otherwise, it can leak and lean out mixtures when hot.

Since the fuel system has not been tested for pressure and fuel volume, I would test both.  When making your Tee and testing the TBI regulator setting, you can temporarily plug or pinch off the return line to the tank and get a quick fuel pump full pressure test.  (Do not run the engine and fuel system with the return line plugged or pinched off for more than just the time necessary to get a fuel pressure gauge reading.)  You can also use the Tee to run fuel into a safe steel container away from the hot engine and exhaust.  Measure the fuel volume and pressure, each is important.

See what turns up...Let us know.

This is my first jeep and honestly want to love it but I'm frustrated.  

 

 

 

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  • 2 weeks later...

 

Before I shove this off a cliff... I need help please.  I bought a cheep 90 wrangler 2.5 tbi 5 speed with a crate engine in it.  It will run perfectly fine for the first few miles (I've driven up to about 10) before it starts losing power, bucks and backfires.  I can shut it off for a bit and then drive again just fine for a short bit then it acts up again.  It seems to idle high as well (1100ish) and sounds cammed when first warming up.  I have some experience working on cars but not a ton.  So far I have:

 

Fuel related:

1.  Dropped the tank to check and see if the soft lines are pinched

2. Checked the tank (plastic) for debris and ran a magnet in to get a small amount of rust out

3. Replaced the gas cap, fuel filter, and fuel pump relay

4. Blew through the lines to make sure they are not clogged 

5. insulated fuel lines near the exhaust

6. installed new injector 

but I can NOT check fuel pressure at the test valve because its rounded out...Yes, you can check the pressure.  Make a test Tee to fit at the inlet line to the TBI unit.  Run a fuel pressure test gauge off the Tee.  Since the pressure regulator should hold pressure at 14-15 PSI under normal conditions, the inlet pressure near the TBI unit should indicate that pressure...Consider this:  The regulator allows fuel pressure above 14-15 PSI to return back to the tank.  This drops the fuel pump pressure reading at the inlet to the TBI unit and reflects the pressure regulator setting...In light of this, make certain that the return line to the fuel tank is unrestricted and that the rollover valve(s) and vent lines to the fuel filler are not restricted.  Lines must be in correct order, including the EVAP system.

First off thank you for replying.  That makes sense with making a T to test on the inlet, I will have to figure that out.  I thought about taking off the throttle body anyway and replacing the gasket which you mentioned later.  With this off I can safely drill out the test screw and replace with a schrader valve to test... I think.

Check the EVAP solenoid function and also the EGR function.  Each can be tested with a simple vacuum pump.  (Search under "EGR" and "solenoid" at our forums, we have been over this on 2.5L engines several times.)  Like other parts, the EGR can stick when hot, especially if carbon has built up.  Check the EGR function cold and hot without burning your fingers!

In regards to the EGR and EVAP, it doesn't look to be hooked up.  I KNOW that the tank vents/rollover valves come together and then go back up to the front of the jeep and just dumps to atmosphere.  It does NOT go to the charcoal canister.  I also know that the EGR is not connected to vacuum.  Can the jeep run okay (although smelly) without this system hooked up?

Spark related:

1.  Changed the plugs, wires, cap, coil, and rotor button—Not a bad call on these parts, the symptom you describe can include a hot, defect coil, the ignition module or even the ECU—especially at higher mileage.  The ECU can also fault when warmed up or hot.  I see that you replaced the ECU as well.  You also replaced the ignition module, reasonable enough, though these parts can be tested before arbitrarily replacing them.  I will note that there's nothing wasted when replacing high mileage electrical or electronic parts.  If they haven't failed yet, they will.  This is just not the best troubleshooting technique.

I actually just tried today replacing the ECU and it ran the exact same (terrible).   I do agree that its not the best technique, but I do return a lot of the parts that do not fix the issue if the parts store lets me.  Again I'm not a great mechanic lol

A factory service manual is a must, especially for any 2.5L model with TBI.  I use a 1989 Mopar factory print manual for 1987-90 YJ Wrangler and 1984-90 XJ Cherokee 2.5L TBI work or diagnostics.  The manual is packed with service data for testing each of the fuel and spark management system components.  A manual is ultimately far less costly than a parts replacement approach.  You can find a CD version at eBay, and a PDF/CD reprint with bookmarks is much easier to navigate and handle than the original print manuals.

I will look for one, thank you.

 

2. Changed the ignition control module

 

Sensors replaced:

1.  Crank position sensor

2. Intake coolant sensor

3. MAP sensor

4. Intake temp sensor

5. O2 sensor

6. CHECKED the TPS—The TPS can test good cold then fault when warmed up.  Test TPS values with the engine cold then hot, ideally when it acts up.  The TPS (throttle position sensor) is like a potentiometer or rheostat.  Voltage increases and decreases according to the movement and position of the throttle valve.  It is not uncommon for the TPS to fault or fail by losing calibration as it heats up.  This device cycles incessantly over time and wears accordingly, aggravated by engine heat and vibration.  We bought our '99 XJ Cherokee 4.0L at 94K miles with all original parts.  By 140K, the TPS was done and needed replacement.

Well when I say I tested it I just moved the throttle and saw that the voltage changed.  I have seen where you have a base reading and then need 17% of that for the other terminal so you adjust it to get that number?  Is that correct?

 

One thing I would add to the TPS possibility is electrical resistance.  This can be anything from a defective alternator, bad grounds (notoriously the engine to firewall ground and others on your YJ model) and even a defective battery or poor battery terminal connections.  Think in terms of junctions and other sources of resistance.  If you have in inexpensive non-contact (infrared) thermometer, do some underhood heat checks at the alternator, manifolds, intake system and the grounds.  Run a simple lamp load test on the ground circuit(s) to confirm wire and terminal integrity.  The YJ Wrangler firewall ground is always suspect, especially when hot.  D.C. 12-volt wiring and connections are as important on the ground circuits as the hot or positive side.

I am going to be honest, I don't know anything about grounds, I don't know how to tell if a ground is bad.  I know the strap you are referring to, how do I test it??  I do have a laser thermometer, would it show excessive heat if its a bad alt?  

 

I also replaced the thermostat and the coolant is always in check.  A compression test (cold and dry) showed 150 to 157 so that seems fine.—Compression is very acceptable and, better yet, well within the 10% limit between high and low readings.  A leak down test would be more precise, but for purposes of tuning and driving, you're well off.  If there is little or no oil consumption, no knocks and good oil pressure, you have a lot to work with here.  Manifold vacuum at idle should be normal, check it to be sure.  Make sure the intake manifold is sealing well.  Otherwise, it can leak and lean out mixtures when hot.

Bear with me, but I don't know how to check manifold vacuum, I have only sprayed carb cleaner around hoses.  I do have a crappy vacuum tester that Ive never used.

 

Since the fuel system has not been tested for pressure and fuel volume, I would test both.  When making your Tee and testing the TBI regulator setting, you can temporarily plug or pinch off the return line to the tank and get a quick fuel pump full pressure test.  (Do not run the engine and fuel system with the return line plugged or pinched off for more than just the time necessary to get a fuel pressure gauge reading.)  You can also use the Tee to run fuel into a safe steel container away from the hot engine and exhaust.  Measure the fuel volume and pressure, each is important.

See what turns up...Let us know.

 

What would I expect to read when quickly plugging the return line if 14-15 is normal ?Thank you so much for your help.  Bear with me I'm learning, but I'm not dumb lol.

 

This is my first jeep and honestly want to love it but I'm frustrated.  

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Frustrated...We'll drill down on your questions and points.  My replies in purple this time:

On 1/25/2022 at 5:31 PM, Frustrated Jeep Owner said:

First off thank you for replying.  That makes sense with making a T to test on the inlet, I will have to figure that out.  I thought about taking off the throttle body anyway and replacing the gasket which you mentioned later.  With this off I can safely drill out the test screw and replace with a schrader valve to test... I think.

If you can do the Schrader valve, great.  That would make pressure testing and flow volume checks much easier now and in the future.  AMC/Jeep should have done this, later engines all have a Schrader valve.

Check the EVAP solenoid function and also the EGR function.  Each can be tested with a simple vacuum pump.  (Search under "EGR" and "solenoid" at our forums, we have been over this on 2.5L engines several times.)  Like other parts, the EGR can stick when hot, especially if carbon has built up.  Check the EGR function cold and hot without burning your fingers!

In regards to the EGR and EVAP, it doesn't look to be hooked up.  I KNOW that the tank vents/rollover valves come together and then go back up to the front of the jeep and just dumps to atmosphere.  It does NOT go to the charcoal canister.  I also know that the EGR is not connected to vacuum.  Can the jeep run okay (although smelly) without this system hooked up?

Make a point of routing EGR and EVAP hoses to OEM scheme.  Check the EGR function with a simple hand vacuum pump and the engine idling.  Open the valve with the vacuum pump, you'll hear an immediate change in rpm if the valve is opening and reseating. 

I always restore EGR for its ancillary benefit:  The EGR cools the combustion process and upper cylinders in an effort to reduce NOx.  This reduces detonation/ping and risk of piston damage.  It's a passive device, nothing lost by running it.

Spark related:

1.  Changed the plugs, wires, cap, coil, and rotor button—Not a bad call on these parts, the symptom you describe can include a hot, defect coil, the ignition module or even the ECU—especially at higher mileage.  The ECU can also fault when warmed up or hot.  I see that you replaced the ECU as well.  You also replaced the ignition module, reasonable enough, though these parts can be tested before arbitrarily replacing them.  I will note that there's nothing wasted when replacing high mileage electrical or electronic parts.  If they haven't failed yet, they will.  This is just not the best troubleshooting technique.

I actually just tried today replacing the ECU and it ran the exact same (terrible).   I do agree that its not the best technique, but I do return a lot of the parts that do not fix the issue if the parts store lets me.  Again I'm not a great mechanic lol

Given time and commitment, you can be a much better mechanic and take this to whatever level you want.  This is where it starts.  

A factory service manual is a must, especially for any 2.5L model with TBI.  I use a 1989 Mopar factory print manual for 1987-90 YJ Wrangler and 1984-90 XJ Cherokee 2.5L TBI work or diagnostics.  The manual is packed with service data for testing each of the fuel and spark management system components.  A manual is ultimately far less costly than a parts replacement approach.  You can find a CD version at eBay, and a PDF/CD reprint with bookmarks is much easier to navigate and handle than the original print manuals.

I will look for one, thank you.

You will take your work to the next level, gain confidence and know you're doing professional grade work with the FSM.

2. Changed the ignition control module

 

Sensors replaced:

1.  Crank position sensor

2. Intake coolant sensor

3. MAP sensor

4. Intake temp sensor

5. O2 sensor

6. CHECKED the TPS—The TPS can test good cold then fault when warmed up.  Test TPS values with the engine cold then hot, ideally when it acts up.  The TPS (throttle position sensor) is like a potentiometer or rheostat.  Voltage increases and decreases according to the movement and position of the throttle valve.  It is not uncommon for the TPS to fault or fail by losing calibration as it heats up.  This device cycles incessantly over time and wears accordingly, aggravated by engine heat and vibration.  We bought our '99 XJ Cherokee 4.0L at 94K miles with all original parts.  By 140K, the TPS was done and needed replacement.

Well when I say I tested it I just moved the throttle and saw that the voltage changed.  I have seen where you have a base reading and then need 17% of that for the other terminal so you adjust it to get that number?  Is that correct?

I adjust the TPS with the throttle closed then check voltage at WOT.  The official/best practice method is outlined in the FSM.  Try to find the FSM in CD duplicate or download at eBay, make sure it's a copy of the Mopar FSM for years 1987-90.  (Any one of those years will work if 2.5L included.)  Details on the TPS test can be found there.  The CD or download is inexpensive and has fast navigation with the PDF bookmarks.  I found this CD (reputable source/producer) for 1987-88, which would work for a 2.5L model with AX5 transmission.  You may want 1989-90, though the content would be similar.  This version is AMC/Jeep, later would be Mopar, look around at eBay:  https://www.ebay.com/itm/282337468496

One thing I would add to the TPS possibility is electrical resistance.  This can be anything from a defective alternator, bad grounds (notoriously the engine to firewall ground and others on your YJ model) and even a defective battery or poor battery terminal connections.  Think in terms of junctions and other sources of resistance.  If you have in inexpensive non-contact (infrared) thermometer, do some underhood heat checks at the alternator, manifolds, intake system and the grounds.  Run a simple lamp load test on the ground circuit(s) to confirm wire and terminal integrity.  The YJ Wrangler firewall ground is always suspect, especially when hot.  D.C. 12-volt wiring and connections are as important on the ground circuits as the hot or positive side.

I am going to be honest, I don't know anything about grounds, I don't know how to tell if a ground is bad.  I know the strap you are referring to, how do I test it??  I do have a laser thermometer, would it show excessive heat if its a bad alt?  

You can test grounds for continuity with a simple ohmmeter.  This is not enough though, because the ground can have continuity but not full contact.  The best test, professionally, is a relatively simple lamp load test.  (I can detail if you want to take it that far.)  Suffice for your purposes is to disconnect grounds at the terminal ends and clean them thoroughly with Scotchbrite pad.  Paint, corrosion and rust will reduce contact at ground points and this increases resistance.  Include the ECU ground, too.  Be sure you have adequate grounds from the battery to frame, to engine and to body.  The sequence is not as crucial as the load carrying ability of ground wires and cables.  Make sure the gauge is adequate (OEM will do) and, again, clean the contact points.  The engine to body ground is notorious for corrosion, clean that terminal point at the body.

I also replaced the thermostat and the coolant is always in check.  A compression test (cold and dry) showed 150 to 157 so that seems fine.—Compression is very acceptable and, better yet, well within the 10% limit between high and low readings.  A leak down test would be more precise, but for purposes of tuning and driving, you're well off.  If there is little or no oil consumption, no knocks and good oil pressure, you have a lot to work with here.  Manifold vacuum at idle should be normal, check it to be sure.  Make sure the intake manifold is sealing well.  Otherwise, it can leak and lean out mixtures when hot.

Bear with me, but I don't know how to check manifold vacuum, I have only sprayed carb cleaner around hoses.  I do have a crappy vacuum tester that Ive never used.

If you need a functional, inexpensive vacuum tester/pump combination tool, Harbor Freight can suffice.  They have several grades of testers, take you pick per budget.  Investing in tools that work and will last is a sound approach, a vacuum pump/vacuum tester combination tool is very helpful.

Since the fuel system has not been tested for pressure and fuel volume, I would test both.  When making your Tee and testing the TBI regulator setting, you can temporarily plug or pinch off the return line to the tank and get a quick fuel pump full pressure test.  (Do not run the engine and fuel system with the return line plugged or pinched off for more than just the time necessary to get a fuel pressure gauge reading.)  You can also use the Tee to run fuel into a safe steel container away from the hot engine and exhaust.  Measure the fuel volume and pressure, each is important.

See what turns up...Let us know.

 

What would I expect to read when quickly plugging the return line if 14-15 is normal ?Thank you so much for your help.  Bear with me I'm learning, but I'm not dumb lol.

Logically, you would expect the pressure to rise to the point of the fuel pump pressure.  This is actually a fast test for the fuel pump pressure and is noted in the FSM as the procedure.  The caution is to only do this momentarily, just long enough to get an accurate ready.  The pump should not be run against a blockage for long.

This is my first jeep and honestly want to love it but I'm frustrated.  

 

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