Sexy stroker turned into Bucking Bronco...

[Mgardiner1]

Again, common example of a coil going bad is USUALLY an open circuit after it comes up to temperature. 99% of the times, when a coil "opens", it has -no- ignition and an engine will generally die, and not restart until it cools. All my experiences with bad coils have all been described as above. I won't swear that its not the problem, because this failure mode is a bit different then anything i've ever come across.

Plug wires ground out would lead to a particular cylinder misfire. This is definitly not a random or single cylinder occasionally misfiring. I've run these I6's with 5 cylinders firing and the engine still has plenty of guts to rev up. This particular problem i am experiencing is completely killing ignition, and restricts the engine from revving any higher. If the problem keeps showing up with random attributes, i'll be even more puzzled.

Started off as just 2800rpm, it would not do it cold, only hot.
Last night, it acted up anywhere about 2000rpm with the engine cold (roughly 120-150F, running temp being 192Fish), and had no problems when hot

Things i know:
1. Engine temperature does seem to have an effect
2. When the problem occurs, reducing the RPM results in running correctly, in other words its like a barrier like a rev limiter
3. It seems to be cutting spark/ignition
4. Throttle position MAY have something to do with it, it seems the harder you push the throttle, the worse it occurs.
5. When starting the engine, the idle seems to be a bit rougher then normal for about 2 minutes, not sure if this is caused by whatever is creating the problem

[SilverXJ]

I have seen plenty of engines with a wall like you speak of with a bad coil.

Also, test the TPS.

-Chris

[Mgardiner1]

yeah, i gotta stop relying on these "new" parts and start testings things a little closer. When i had a bad throttle position sensor in the very beginning, the engine acted much different then this failure. The reason i rule out the throttle position sensor, is it doesn't seem to matter if you sneak up to 2800 with a very soft pedal, or if you give 75% throttle, it still flatlines the same way at the same RPM.

[Mgardiner1]

I must admit i'm getting frustrated with this problem. The conditions are just too random, the dead spot always occurs at 2800 with the exception of the one night (like i said, random). Its too damn cold outside to puts around with the thing. Its driveable, i guess i'll just drive it for now and see about getting it in the garage after the holidays....

Please, if anybody has any similiar experiences or any other advice, i'd love to hear from you. Thanks

[gonridnu]

I have also seen this with a coil and generally agree with your open circuit theory on coils but have seen them where they could not recover quickly enough to generate adequate spark above a specific RPM. I have also seen this in plugged exhaust systems where the ability to flow was reduced by either a plugged CAT or in one instance a GM vehicle that had dual wall EX tube where the inner wall had collapsed and reduced the cross section of the pipe significantly enough that it would not allow the vehicle to rev over 2K....just other ideas

[gonridnu]

And in thinking further have also seen this with fuel delivery problems...ie fuel filter and pump problems....not sure how any of these would fit into your downshift and it runs fine one U got past the "wall" scenario but just other things that are fairly easily checked.

[Muad'Dib]

Does the engine backfire at all out the intake or exhaust when this problem is happening?

I had a similar problem in my 90 that was caused by a bad fuel pump. I was running lean.

[Mgardiner1]

I'll see if i can answer all responses here.

I have also seen this with a coil and generally agree with your open circuit theory on coils but have seen them where they could not recover quickly enough to generate adequate spark above a specific RPM. I have also seen this in plugged exhaust systems where the ability to flow was reduced by either a plugged CAT or in one instance a GM vehicle that had dual wall EX tube where the inner wall had collapsed and reduced the cross section of the pipe significantly enough that it would not allow the vehicle to rev over 2K....just other ideas

When i have an opportunity, i'll take the coil out and follow FSM procedures for checking primary and secondary coil resistances. I have no other methods of testing ignition coils other then replacing it. The idea that the fields cannot recover fast enough kinda gets shot down by the fact it works at RPM's above its problematic spot. Also, the pulse from the ECU is reflected in the operation of the tachometer, so that kinda proves the ECU is cutting the ignition for an unknown reason. I changed the ASD relay just to be sure.

The exhaust system is brand new, i mean it, 300 miles on it, and its 3" diameter from the collector back.

And in thinking further have also seen this with fuel delivery problems...ie fuel filter and pump problems....not sure how any of these would fit into your downshift and it runs fine one U got past the "wall" scenario but just other things that are fairly easily checked.

I've verified fuel pressure while standing still under all RPMs. There is never a dip in pressure. The symptoms also seem to be much too sudden to be fuel pressure fading.

Does the engine backfire at all out the intake or exhaust when this problem is happening?

I had a similar problem in my 90 that was cause by a bad fuel pump. I was running lean.

No, the engine simply boggs, and walls like a rev limiter, which is kinda surprising. Since it seems to be loosing spark, i would expect it to load up a fuel charge and ignite in the header. However, when the condition occurs, i upshift right away and don't continue to aggrevate the problem.


Guys the input is great, it really is. I appreciate any and all help i can get. I've got a bit of experience with these engines now (although far from an expert), and i'd consider myself a good mechanic with several years experience. I've read through a lot of the "Theory of operation" in the FSM for a 1993 Jeep, and can't really find anything that would help isolate this problem

Let me throw 2 more things out. I verified a resistance test on the engine coolant temp sensor, and at operating temperature its at 800 ohms like it should be.
I am using a 1991 engine, a 1991 ax15, but i am using the bell housing for a 94 and newer. I wanted the external slave cylinder setup. The only difference i can establish between the crank position sensor from 91-92 and 93-95, is the connectors. However, i do not have any engineering schematics that would give specific dimensions of the sensor depth. How would one go about measuring installed clearance for the CPS?

[Flash]

I suspect its taking the spark and the inj pulse away......and thats why its not back fireing......

If this is true, then it could be the ECU that is at fault........But it could still be a sensor that it's making the ECU do bad things

Do you have a NOID-LITE,(that will fit, to test your inj pulse?) and a spark tester that keep the spark plug in the loop?(so you can run the eng, testing for spark, with out a inducted misfire)
This could be some valuable Data whether is spark, fuel.....or both.

Flash

[Mgardiner1]

Yeah Flash, i have an inline spark tester. I do not have anything to test injector pulse. I bet a O-scope would be great for that. I could also read the CPS sensor activity as well.

[gonridnu]

Finally viewed Ur video and retract all previous guesses except maybe the coil but concur on the CPU not liking something. If it were the CPS it should throw a code so it would be interesting to know if it were spark only or a total shut down by the CPU.

[Mgardiner1]

The video isn't shit either, you should go for a ride and see how it feels when you are on the road! Pulls strong up to the problem zone and hits a BRICK WALL! The ride to work this morning, problem was there. The ride home from work, problem was gone. WTF

Now for something a bit more constructive.....
I think if the ECU was recieving an error from a sensor, it would show up in daignostic trouble codes. HOWEVER.... The ECU is external and in the engine bay, for how cold its been, i can't see how heat soak would effect the ECU, and to only a specific RPM range. As far as the "Theory of operation" is concerned, the ONLY 2 items aware of engine RPM, are Crank Position sensor, and the ECU itself. I'm going to stick with this track as the only consistant part of it, is the RPM that it is problematic at (always same RPM). I did have that one crazy night where the problem zone was much lower, but i also did not allow the engine to warm up at all which i usually do.

Sorry, i posted this after drinking.

[Mgardiner1]

I drove around for 2 hours last night, (me, sis, and bro-n-law, just ridin, trying to find an open bar on xmas eve) I punched it a few times with no problems at all. We found a destination, 3 hours later we head home, and the 2.8K problem was there in the beginning of the drive.

[Mgardiner1]

My jeep is now basically undriveable.

Dec 24th as says above, the truck ran FINE with no problems until the ride home. Dec 26th, i had no issue's going to work, but on my way home from work, it got to the point i couldn't accelerate. I had to pull off to a gas station for a few min, checked under the hood real quick, and was able to limp it home. Now everytime i drive it, i can't even rev over 2000 RPM. The longer the engine runs, the better it runs.

I'm so happy i spent so much money to end up with this

[Flash]

Sounds like it's time for some testing, of components,while it...... it is broken


Flash