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Moses, I made a video of the Snap-on MT2500 scan tool readings maybe this will help in the RPM issue. Here is some history of the 89 Jeep Wrangler YJ 2.5L.  I have what i call a hiccup when idling or driving every once in awhile. The engine is usually warm to operation temp and will hesitate or act like it dies ( rpm drop at an idle to 200 rpm or so) while idling in the drive way. Sometimes will die but will start right back up and there is days when it dose this hiccup while i am going 25 or 45 mph or what ever speed and has no rhyme or reason. 

Here is what I have replace or trouble shoot with a multi-meter (analog and digital meters), i check and replace vacuum hoses where needed and replace 4 small hoses on the TBI.  Clean the crank sensor, replace MAT sensor, installed MAP sensor but ended up not using so replace with original. I compression check each cylinder and had 95 to 100 psi per cylinder. I put Seafoam in each cylinder and let sit overnight. Did this twice to get rid of the blow-by and it hasn't consumed any oil in 1000 miles spark plugs look good too, light colored. The Plastic valve cover was replace with an new oil cap as well because of oil being push out thru the oil cap.

After the Seafoam treatment I check compression and found it went up to 110 in each cylinder. The engine has no leaks and no noise of any kind that would point towards replacing it.  Also put Seafoam in the gas and poured some into the TBI. This seem to work quite will for smoothing out the idle and performance. I have real good acceleration. Previous owner replace the spark plugs, wires, rotor and cap, O2 sensor, belts, and change all drive train fluids to synthetic and oil change with Valvoline 10-30. I Check grounds under hood and they look great.

The engine idles about 800 rpm. I borrowed a Snap-on MT2500 and scan it but really nothing stands out. When the jeep is running I wrote down the readings from the MT2500 and notice the ST Fuel trim 126 and LT Fuel trim 133 steady. I listen to the TBI with the air horn off and sounds like a hissing noise when the engine starts to hesitate or stall, but will start right up. Kind of out of answers other than the TBI needs replace and I have plenty of power on the highway and thru the whole power band from start to 80 mph.

I removed the gas tank because the sending unit was showing incorrect gas level so I tested the gauge and the sending unit and found the baffle in the tank was the issue, so I removed it and check all the fuel lines and the vents and all is good and not plugged up. Gas gauge reads correct now and it seems to run pretty good today as I drove it for about 10 miles.  What is your thoughts, thanks.

VID_20161029_134408091.mp4

VID_20161029_134128876.mp4

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mchardtke...Below in red text is my reply to your thoughtful note:

11 hours ago, Mchardtke said:

 

Moses, I made a video of the Snap-on MT2500 scan tool readings maybe this will help in the RPM issue.

Sorry, could not view the .mp4 files as posted...We can discuss whether they are posting correctly.

Here is some history of the 89 Jeep Wrangler YJ 2.5L.  I have what i call a hiccup when idling or driving every once in awhile. The engine is usually warm to operation temp and will hesitate or act like it dies ( rpm drop at an idle to 200 rpm or so) while idling in the drive way. Sometimes will die but will start right back up and there is days when it dose this hiccup while i am going 25 or 45 mph or what ever speed and has no rhyme or reason.

Sounds like a fuel supply (pump or ECU) issue...Have you checked the ECU connectors and contacts?

Here is what I have replaced or trouble shoot with a multi-meter (analog and digital meters), i check and replace vacuum hoses where needed and replace 4 small hoses on the TBI.  Clean the crank sensor, replace MAT sensor, installed MAP sensor but ended up not using so replace with original. I compression check each cylinder and had 95 to 100 psi per cylinder. I put Seafoam in each cylinder and let sit overnight. Did this twice to get rid of the blow-by and it hasn't consumed any oil in 1000 miles spark plugs look good too, light colored. The Plastic valve cover was replace with an new oil cap as well because of oil being push out thru the oil cap.

Did you check ohms resistance and continuity of the crank sensor?  Check the crank sensor wire(s) for opens or high resistance?  Seafoam certainly made a difference.  Sounds like either carbon on the valve seats and faces or more likely gummed/sticky piston rings.  Rings may be sticking in their grooves.  Compression loss was uniform, which mean a "class" problem like gummed rings.  This can happen from the vehicle setting up too long, poor oil change maintenance or switching to synthetic oil at high mileage.  Synthetic oil in an engine run for a long time on conventional oil can cause carbon and gum around the rings to dissolve.  This causes the rings to stick in the piston grooves.  The blow-by is a hint that the rings were not sealing against the cylinder walls.  

I would run a cylinder leak down test on each cylinder with its piston on TDC of the compression stroke.  Here is a post I did on leak down testing and why it is superior to a conventional compression check:  

http://forums.4wdmechanix.com/topic/75-engine-diagnostics-compression-gauge-versus-a-cylinder-leak-down-tester/

After the Seafoam treatment I check compression and found it went up to 110 in each cylinder. The engine has no leaks and no noise of any kind that would point towards replacing it.  Also put Seafoam in the gas and poured some into the TBI. This seem to work quite well for smoothing out the idle and performance. I have real good acceleration. Previous owner replace the spark plugs, wires, rotor and cap, O2 sensor, belts, and change all drive train fluids to synthetic and oil change with Valvoline 10-30. I Check grounds under hood and they look great.

110 PSI is not very high for a 2.5L TBI engine, but the good news is that the cylinder pressures are uniform.  Especially on a four-cylinder engine, you want PSI within 10% between the highest and lowest cylinder.  I'm okay with an engine that is not blowing by oil and "feels strong" still, regardless of actual PSI per cylinder.  With the ignition disabled (coil high tension wire removed), did you hold the throttle open wide during the compression test cranking?  That's necessary for correct readings...What is the manifold vacuum reading at an idle?

The engine idles about 800 rpm. I borrowed a Snap-on MT2500 and scan it but really nothing stands out. When the jeep is running I wrote down the readings from the MT2500 and notice the ST Fuel trim 126 and LT Fuel trim 133 steady. I listen to the TBI with the air horn off and sounds like a hissing noise when the engine starts to hesitate or stall, but will start right up. Kind of out of answers other than the TBI needs replace and I have plenty of power on the highway and thru the whole power band from start to 80 mph.

The hissing sounds like a cut off fuel supply.  Could be intermittent fuel starvation from a faulty pump and/or pump ground.  

Here's a technique that isolates an ignition problem from a fuel supply problem on a TBI system:  Attach a strobe timing light to #1 cylinder spark plug wire.  With the air intake hood removed from the TBI unit, run the engine while holding the timing light above the air horn, the light beam aimed at the fuel injector.  You will accomplish two tests:  1) If there is an ignition fault, the light beam will stop momentarily or "hiccup" as you say; and 2) if there is an injector or fuel flow problem, you will see the fuel flow reduce or stop spraying.  This is a quick and fail safe test for both ignition and fuel.  This visual test can only be done on a TBI engine;  port or MPI injection is not a visible fuel spray, the spray is within the intake ports.  

You can also see the quality and force of the injector spray pattern in this way.  The spray pattern should be conical.  You may need to dim the room light to produce a good image of the timing light's illumination of injector pulses.  The spray cone should look steady at engine idle speed, uniform in appearance.  

You'll like this test technique.  As the engine hesitates, or ideally drops to 200 rpm, you will see either the stoppage of timing light flashes (signaling ignition trouble) or no fuel supply (the fuel spray cone stops while the ignition continues to fire the timing light). 

I removed the gas tank because the sending unit was showing incorrect gas level so I tested the gauge and the sending unit and found the baffle in the tank was the issue, so I removed it and check all the fuel lines and the vents and all is good and not plugged up. Gas gauge reads correct now and it seems to run pretty good today as I drove it for about 10 miles.  What is your thoughts, thanks.

Could have been a pump fuel pickup issue if the pickup sock was dirty.  If the symptom returns, try my timing light troubleshooting technique.  Try it anyway, it's very interesting!

Moses

 

 

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Moses, thanks for your time and valued advice,  I will run the tests you provide me and let you know what I fine. Quick question on the RPM difference between the Snap-on MT2500 and the Jeeps RPM gauge of up to 500 RPM (example: Gauge 2500 rpm - Snap-on MT500 RPM 1768 RPM any advice, and thanks again your articles are very informative and just as useful !!!

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Mchardtke...Review my exchange at the forum topic and postings below.  Try a simpler tach-only tester on #1 plug wire to read actual spark pulses.  The ECU should not impact the spark pulses if the engine runs smoothly and does not seem to misfire, ping/detonate or backfire.  A direct pulse read with a tach that picks up the spark plug current by induction is the most accurate read.

The MT scanner should read correctly if it relies on a #1 plug wire signal.  But you believe the gauge is more accurate. A tach-only tool with #1 plug wire pickup should solve the mystery.  The ignition fires only at TDC of each cylinder on its compression stroke.  There are motorcycle engines that fire at TDC on the compression and exhaust strokes, but not the Jeep 2.5L four cylinder engine.

Read the exchange below, and see if that helps.  If the ignition module or ECU were defective and scattering the spark trigger, you would have misfire, backfire and other signs of spark timing errors.  A tach signal is typically from the primary side of the coil or, in the case of fuel and spark management, from the CPS and ECU functions.  I would separate issues:  engine performance being one concern, tach/gauge readings being another concern.

I will try to open your .mp4 files this weekend, I'm at the SEMA Show using my laptop with a Wi-Fi hot spot.   

 

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Mchardtke...I'm home and able to view your two .mp4 files.  Thanks for the detail, they're helpful.  

Just from my ear, it sounds like the Jeep tach is more accurate than the Snap-On MT2500 scan tool for rpm.  Do you have access to another scanner of this type?  If so, it might be worth a comparison.  I'm wondering whether the scanner's tach/#1 pickup software calibration is for a Jeep 4.2L six?  Any updates on your tach readings?

Moses

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IMG_20161106_164826448.jpgIMG_20161106_164723876_HDR.jpgIMG_20161106_164744721_HDR.jpgIMG_20161106_164819286.jpgIMG_20161106_164826448.jpgMoses, I do have a Snap-on tech that I had to do some work on and now it is working, I will let you know what I find, Thanks. By the way here are some pictures of the Jeep, I had to fabricate a  battery hold down which turned out OK. I still get a slit oil leak from the new factory oil cap so I believe I will put some more Seafoam into each cylinder to see if there is still some ring or carbon issues that may be causing some undo cc pressure. I haven't had time to do a leak down test yet still working on other areas like the rear track bar, that was fun to remove on the passenger side. PS here is what the spark plugs look like as well thanks for you help and time I really it, talk to you soon.

IMG_20161031_094410868.jpg

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Mchardtke...I really like your YJ Wrangler!  So seldom that we see such a clean and original model.  Great photos...Thanks for sharing!

I would run the engine with the filler cap removed and your hand firmly on the filler cap opening of the valve cover.  Feel the pressure and any "pulsing" at idle.  Years ago, we had an inexpensive gauge that could be placed here to read blowby pressure for emissions testing.  You could simulate that with an old oil filler cap drilled and tapped for a hose nipple and a hose attached to a simple vacuum/pressure gauge...The idea is to see if there is excessive pressure.

Your PCV valve may not be working properly.  Make sure there is adequate vacuum and flow at the suction side of the valve and that the valve opens properly.  If stuck or restricted, this could prevent normal purging of both pressure and fumes from the crankcase.  Excess pressure, as you hint, could be the result of piston ring blowby to the crankcase.

Moses 

 

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Moses, thanks I knew if anyone would appreciate this Jeep Wrangler besides the wife and myself would be you. What CC pressure would you expect to see and I do have one of the PCV picture below and at an idle it is in GREEN SAFE and when riv the engine it go's into the DANGER RED.  Also I found my ISA tester to activate the forward and reverse of the motor. I found I had a growling noise at the ISA motor so I order one from RockAuto ( which is AWESOME) and will replace and adjust the ISA motor for full extension at 3500 rpm. Sorry about the jumping from one subject to another I thought you may be interested, thanks again Moses and look forward to hearing from you!!

images.jpg

IMG_20161108_162531308.jpg

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Mchardtke...OMG, I haven't seen a gauge like yours for decades, exactly what I was describing!

Your crankcase and PCV are working.  The Lenroc PCV Tester was described in the April 1965 issue of Popular Science as:

"Automatic smog-valve tester...This simple device checks any PCV (positive crankcase vent) system in seconds:  just place it over the oil-filler hole when the engine is idling. A movable ball will indicate either a slight crankcase vacuum (valve okay) or pressure (something is wrong).  $3.98.  Lenroc Co., S. San Francisco, Calif."

Your tests at engine idle are good, this is the only speed to test with the gauge.  If there is piston ring blowby, with the PCV valve and hose removed, rev the engine up and down.  You will see oily (blue) smoke puffing out the valve cover at the oil fill hole or the PCV grommet/hole.

Significant blowby will even show with the engine idling, a puff-puff in rhythm with the cylinder(s) blowing gases past the piston rings.  You will usually have significant oil burning/consumption if the puffing is steady and looks like burnt oil and gases that bear a faint gasoline smell.  If not a significant volume of blowby, these crankcase fumes will get drawn through the PCV valve under normal operating conditions.  You will not see smoke, even at the oil fill cap hole, with the PCV valve and its vacuum hose in place.

Moses  

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17 hours ago, Mchardtke said:

Moses, also there is suction when I put the palm of my hand over the oil filler cap hole at an idle.

As viewed, the device atop the rear of the valve cover would be a vacuum source.  Its hose attaches at the rear of the intake manifold, right?  This would account for the suction at the oil filler cap hole at an engine idle.  The PCV valve is closed at high manifold vacuum (engine idle); the suction source would originate from the back of the valve cover at this device...Sound correct?

Moses

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Yes, that is correct. Sorry I have been out of town so I couldn't get back to you sooner. There is no markings on the ISA test tool so I would say it is aftermarket, I tried it on the ISA and to works great. Well all seems to be working well I haven't had any Hiccups sense I worked on the gas tank and went to 87 octane. I really don't know if it was a gas issue or an ejector issue but no more issues knock on wood. Thanks for your input and I will keep you posted as things progress and thanks again for your great advice Moses take care.

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You're very welcome!  Winter ahead, this was a good time to get the Jeep YJ in good, reliable working order...Keep us posted, looking forward to your posts!

Moses

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