1990 Jeep Wrangler 2.5L TBI 4-Cylinder No Start

[ShawnT]

My son just purchased a 1990 Wrangler with a 2.5 4 cylinder.  We noticed that it had oil leaks and coolant leaks that needed to be addressed.  So we replaced the radiator hoses, heater hoses, heater core, thermostat, and thermostat housing.  As we were bringing the Jeep up to temperature to ensure there were no leaks from the hoses we had just replaced, the engine died, as if it had ran out of fuel.  The gauge read 1/2, but to be safe, we added fuel and tried to start it. No start.

I checked for spark, and we had spark.  I pulled the spark plugs out of the engine and cranked the engine over.  I noticed a large amount of fuel pouring out of the back cylinder.

I checked this site for values on different sensors so that I could check them one by one.  I tested the MAP sensor, voltage read 5 volts in, 4.85 volts out.  I tested the TPS, seemed to test OK.  I unplugged the fuel injector that rests on top of the throttle body.  I ohm'd it, got a reading of 2.5 ohms.  I purchased a remanufactured injector from the parts store and ohm'd it, which I received the same 2.5 ohm reading.  I replaced the injector with the remanufactured one, no start.  I unplugged the injector and the engine started for a few seconds, presumably from the fuel that was left in the throttle body.  I could repeat this by keeping the injector unplugged and using starting fluid to run the engine.  Seemed to run fine. 

Being absolutely stumped, I read about the MAT sensor acting similarly to a Mass Air Flow sensor.  I replaced the MAT sensor with one that Car Quest cross referenced as being the right one.  The sensor itself looked right, but the connector was different. I purchased a new pigtail and wired it in.  After doing so, the Jeep started right up and ran.  I drove it for about 5 miles before it died. Same symptoms: seemed to be over fueling.  I read more information about the Coolant Temperature Sensor, which I was able to finally locate it on the under side of the intake manifold.  I noticed the CTS had bare wires near the sensor. I ohm'd the CTS which ohm'd out at 8,000 ohms.  I ohm'd the new part which ohm'd at 6,200.  I replaced the CTS along with the MAT sensor (which I had warrantied out at Car Quest).  The Jeep started right up. I drove the Jeep for approximately 5 miles with no issues.  I started it up the next morning.  I thought everything was fixed until I went to start it up later in the day.  It failed to start again.  Same symptoms: over fueling.   I thought the CTS with the bare wires was the smoking gun.  Now, i'm not sure.  It seems like both times that I have replaced the MAT sensor, it ran great for 5 miles or so then back to the same problem.  I ohm'd the second MAT sensor after the no start.  It ohm'd out at 18,500 ohms.

I'm at a loss at what to do from here.  Maybe an ECU issue? 

Any help would be greatly appreciated!

 

Shawn

 

[Moses Ludel]

See my comments in red highlight below, Shawn T.  I would start here:

11 hours ago, ShawnT said:

My son just purchased a 1990 Wrangler with a 2.5 4 cylinder.  We noticed that it had oil leaks and coolant leaks that needed to be addressed.  So we replaced the radiator hoses, heater hoses, heater core, thermostat, and thermostat housing.  As we were bringing the Jeep up to temperature to ensure there were no leaks from the hoses we had just replaced, the engine died, as if it had ran out of fuel.  The gauge read 1/2, but to be safe, we added fuel and tried to start it. No start.

I checked for spark, and we had spark.  I pulled the spark plugs out of the engine and cranked the engine over.  I noticed a large amount of fuel pouring out of the back cylinder.

I'm guessing that you have spark at all four cylinders?  The #4 cylinder spark plug is obviously fouled with that kind of fuel pooling.  Check the spark at this spark plug lead, using a fresh or clean spark plug.  If good spark is present there, I would be concerned about low compression.  If weak, test the spark wire for ohms-resistance.  

A quick compression test would be smart.  A cylinder leak down test would be more definitive.  Use the search box at these forums to find my comments on the use of a leak down (or leakdown) tester.

I checked this site for values on different sensors so that I could check them one by one.  I tested the MAP sensor, voltage read 5 volts in, 4.85 volts out.  I tested the TPS, seemed to test OK.  I unplugged the fuel injector that rests on top of the throttle body.  I ohm'd it, got a reading of 2.5 ohms.  I purchased a remanufactured injector from the parts store and ohm'd it, which I received the same 2.5 ohm reading.  I replaced the injector with the remanufactured one, no start.  I unplugged the injector and the engine started for a few seconds, presumably from the fuel that was left in the throttle body.  I could repeat this by keeping the injector unplugged and using starting fluid to run the engine.  Seemed to run fine.

Sounds like too much fuel pressure.  Check the pressure regulator's pressure setting at the TBI test port.  You want 14 PSI or so, no more than 15 PSI.  Too much pressure could cause the problem you're seeing.  I'd also test the actual fuel pump pressure. (Test the maximum, you'll find references to the pump and pressures in these forum topics on Jeep 2.5L TBI troubleshooting.)  Be sure that the fuel pump has not been replaced with a higher pressure MPI type pump. 

Being absolutely stumped, I read about the MAT sensor acting similarly to a Mass Air Flow sensor.  I replaced the MAT sensor with one that Car Quest cross referenced as being the right one.  The sensor itself looked right, but the connector was different. I purchased a new pigtail and wired it in.  After doing so, the Jeep started right up and ran.  I drove it for about 5 miles before it died. Same symptoms: seemed to be over fueling.  I read more information about the Coolant Temperature Sensor, which I was able to finally locate it on the under side of the intake manifold.  I noticed the CTS had bare wires near the sensor. I ohm'd the CTS which ohm'd out at 8,000 ohms.  I ohm'd the new part which ohm'd at 6,200.  I replaced the CTS along with the MAT sensor (which I had warrantied out at Car Quest).  The Jeep started right up. I drove the Jeep for approximately 5 miles with no issues.  I started it up the next morning.  I thought everything was fixed until I went to start it up later in the day.  It failed to start again.  Same symptoms: over fueling.   I thought the CTS with the bare wires was the smoking gun.  Now, i'm not sure.  It seems like both times that I have replaced the MAT sensor, it ran great for 5 miles or so then back to the same problem.  I ohm'd the second MAT sensor after the no start.  It ohm'd out at 18,500 ohms.

Check the fuel pressure at the regulator test port on the TBI unit.  Test fuel pump maximum pressure.  The ECU, of course, could be an issue, but I would start with fuel pump and regulated pressure at the TBI unit.  The #4 cylinder compression should be confirmed as well.  We can go from there...

I'm at a loss at what to do from here.  Maybe an ECU issue? 

Any help would be greatly appreciated!

 

Shawn

 

 

[ShawnT]

I forgot to add that into my trouble shooting description.  I changed the spark plugs with Champion plugs.  I also checked the fuel pressure at the main fuel line coming into the TBI, which read approximately 50 psi.  I then tested the fuel pressure at the TBI test port which read 14-15.  I will go check compression on all four cylinders and report back.  Thanks for the quick response!

 

Shawn

 

[Moses Ludel]

Shawn...50 PSI is high.  See the pump specs at this forum question:  

Testing should only be momentarily, as pressure builds rapidly and distorts the results.  Your pressure should read a much lower peak from the pump to the TBI unit:  more like 17 PSI.  You may have a high pressure MPI pump in the tank.  This would overload the TBI.  

Note: Before dropping the tank and checking the pump type, inspect the return flow from the TBI regulator to the fuel tank; return flow must be unrestricted for the fuel pressure to regulate and read properly. Otherwise, you can get a spike reading from a lower pressure TBI pump that makes it seem like you have a high pressure MPI pump.  Inspect the fuel return line and be certain that the fuel tank is venting properly through the vapor canister.  A mis-routed set of fuel tank and EVAP hoses, or poor tank venting for any reason, can be another way that the fuel return flow from the TBI unit gets restricted.  When the pump is running, excess fuel should be returning to the tank.

Before running the compression test, see my response a few minutes ago to our fellow member with the RAV4 Toyota oil burning issue.  I talk about the value of a cylinder leak down test. If you suspect wear or want a true picture of the engine's condition without removing the cylinder head, consider a leakdown test.  I've commented at the forums about these tests.  Use the search box (set the search box for "All Content" and use key words "leak down" or "leakdown") for more details.  A compression gauge is a rough quick test, try it first.

Moses

[ShawnT]

Ok, I checked the engine compression before I saw this response.  I found cylinders #1, #2, #4 to be 120 PSI. Cylinder #3 was 130 PSI.  I haven't performed a leak down test yet.  I also checked the fuel return line from the TBI to the tank by removing the fuel tank filler cap and inserting compressed air at a decent rate from TBI side, using an air hose.  My son was on the fuel tank end stating he could easily hear the flow of air.  I know, not super scientific, but seemed to work.

[ShawnT]

So another weird question. Each time I have replaced the MAT sensor, the engine started right up and ran seemingly normal for approximately 5-7 miles and then died.  Since I installed a Car Quest aftermarket part, do you that its getting ruined quickly because of a different issue?  or possibly the wrong sensor?  Doesn't make sense to me.

[Moses Ludel]

Good question, ShawnT...MAT/IAT sensors are not complicated, they are likely a wax-pellet or similar design much like a coolant temperature sensor.  As you know, there is a relationship between incoming air temperature (Intake/Manifold Air Temperature) and the ohms resistance at the switch/sensor terminals.

First off, without being anecdotal, I just had a discussion with "crazy for the 4.7" about OEM versus aftermarket off-shore parts:  

Parts suppliers like CarQuest, AutoZone and, sadly, even NAPA have gone for maximum profit and low price points on many items.  (Historically, if you bought Echlin from NAPA, you got premium OE-grade.  Echlin was unbeatable for quality and reliability.)  In an effort to be price competitive, NAPA and others have lowered the bar or now offer "standard" grade parts that really mean stuff like AutoZone or CarQuest might sell:  typically offshore sourced.  Without bashing retailers, this is the nature of the automotive aftermarket, and even OEs, in the wake of declining OE parts made onshore.

The other issue with non-OEM parts is general application of these parts.  A particular MAT, MAP, TPS or O2 sensor might fit a range of applications, but to do so requires more universal, supposedly still in tolerance calibrations.  I generally note how many vehicle types and applications a particular brand-X part will fit.  Compare that to how many applications the OE part will fit.

If skirting the OE (in this case it would be Mopar, which may no longer carry an older application part), I try to determine who supplied the part(s) to Mopar in the first place. Example:  For an O2 sensor like my '99 XJ Cherokee 4.0L application, I took the NTK number from the OE sensor and found an NTK sensor under that part number.  Simple.  I'm still getting NTK quality and a specific application/calibration.  By contrast, I tried a Brand-X replacement sensor that did not perform properly.  This is not a slight to the other source, but it does illustrate the universal nature of many aftermarket parts, parts that likely do work on some application.  Here are the aftermarket O2 sensors for just the 1999 4.0L Jeep engine, you could run each of these numbers and see how many applications each sensor fits:

http://www.rockauto.com/en/catalog/jeep,1999,cherokee,4.0l+l6,1431776,emission,oxygen+(o2)+sensor,5132

Do the same thing for your IAT/MAT sensor.  See what you discover about the CarQuest part.

Yes, you might have an IAT/MAT calibration problem or even cheap parts, but I would also consider a contamination problem.  There was an indication that "fuel" was pooling at #4 cylinder.  Did you make sure that the #4 spark plug is firing properly, that the plug and plug wire lead are okay?  Fuel or air/fuel mixture passes through the intake manifold on a TBI engine, just like a carbureted engine.  Is the new IAT/MAT sensor getting contaminated?  With fuel?  Coolant?  A mix of fuel and coolant?  What is the contaminant?  Confirm what is actually pooling at #4 cylinder.  

Look over the 2.5L TBI engine's intake manifold parts schematic and the location of the sensors.  Is there a possibility that the IAT (MAT) sensor is in a coolant port instead of the air passageway?  Could the intake manifold be leaking or have a hairline crack into a coolant passage that opens up with engine warming?  Are you losing coolant?  Look this over:

 1987-90 2.5L TBI Intake Manifold and Sensors.pdf

Zoom into the PDF for details...

Moses

[Moses Ludel]

ShawnT...I did want to comment that the 120, 120, 130, 120 psi compression is adequate and suffices as a compression gauge quick check.  The best part is that the high cylinder is less than 10% above the lowest cylinder(s).  That's good enough for cylinder balance.

If you have access to a cylinder leak down test, you can pinpoint the wear and also the degree of wear.  I reserve comment until you have those figures...We can discuss the results.  

Moses

[ShawnT]

I found the problem, two bare wires behind the TBI.  It appears the wires were rubbing through on the intake manifold and shorting out.  Luckily I had my head in the engine compartment as my wife was cranking the engine over.  I saw an orange glow, which led me to investigate further.  It appeared someone had repaired the wire before and did a poor job, leading to a poor routing angle.  I replaced the bad section of wire, inspected the rest of the wires in the connector that plugs into the TBI and rerouted it. 

On a side note:  Does anyone have a good solution to sealing the plastic valve covers?  When I purchased this Jeep, the previous owner said he had just replaced the valve cover gasket with a OEM O-ring style gasket that fits inside of a channel that is grooved into the valve cover.  It was leaking pretty severely. I replaced the valve cover gasket again with a Fel-Pro rubber flat style gasket, using a thin layer of Permatex (for added insurance) with the same, oil leaking result.  I was meticulous in cleaning the head surface and the valve cover, as well as inspecting it for any cracks or anomalies.  I'm hoping someone has some experience with these plastic valve covers. 

[Moses Ludel]

ShawnT...Congrats on the discovery, this finding was sheer timing!  Would have involved a lot of troubleshooting otherwise...

As for the leaking plastic valve covers, the key is whether they are warped or not.  Check for straight on a granite slab or sheet of safety glass.

Tightening valve cover hardware is another concern: over-tightening will warp the lip(s).  Also, the leak could be a clogged ventilation system causing excessive crankcase pressure.  Look over the exchange with Sinisterwillys40 at: 

I included a vacuum circuit diagram and emissions hose schematic at that exchange...Trust it will be helpful.  The parts orientation is important.  You may have improperly routed hoses or incorrectly installed devices like the ventilation filter.  The PCV valve could be clogged or not getting proper vacuum.

Moses