Engine miss above 2000 RPM

Just dawned on me that valves can be sticky, seal one minute and leak a little or stick the next, and be slow to completely seal and close, and could pass a compression test, or presumably even a leak down test?

A Cat going bad, and increasingly blocking the exhaust would likely to make the engine tend to run rich (too little air supply) I think, and would match the low STFT numbers I was getting.

At idle the STFT was low trying to compensate for an apparent rich exhaust, while it idled smoothly, in closed loop and showed no other signs of a blocked exhaust. A low STFT in closed loop means the ECU is sending less fuel (shorter injector open time) that the data tables suggest for the other sensor data.

But an O2 sensor reporting rich exhaust (low O2 concentration) when the ECU used data tables for the fuel feed rate, and then leaned out the fuel feed to keep the O2 sensor-ECU operating in closed loop, thus the low STFT numbers, would not be from incomplete fuel burns as that would leaving excess O2, making the O2 sensor-ECU controls think it was running lean, which is not what a low STFT says?

In other words, a low STFT suggests either too little O2 (rich), or too much fuel(rich)!!!! And suggests the ECU is compensating with less fuel supply to raise the amount of unused O2 in the pre-cat exhaust???

If I am right that suggests based on all the data, that it could not be incomplete fuel burning in the engine as that would leave excess O2(lean), so it must be a blocked exhaust or intake (or valve problems), or combination.

But the original low STFT data, before the max RPM fell from 3000 rpm to 500 rpm, was when I had a suspect MAP sensor data stream (Vacuum hose leaks to the MAP sensor found and fixed just before the peak rpm and idle crapped out). And I am not sure I could see any STFT, or even closed loop operation data now, now as it barely runs at 300-500 rpm, puking all the time and trying to die.

Just thinking out loud here, trying to make sense of all the data I have.

Am I missing anything? Is this logic sound????

While this was in interesting mental exercise, I am still back to checking the Intake and exhaust for a restriction, Bad Cat? And swapping out the Cap, rotor, plug wires and CPS as the only remaining parts I have not swapped out, short of pulling the head now.

Just had a thought, I think the vacuum gauge readings were pretty steady at about 10-11 " of vacuum when the engine is miss firing, first at 3000 rpm, and now all the time at 300-500 rpm.

If my memory is correct, the fact that it was steady(?) and low, with good compression confirmed, would imply a bad Cat Converter, blocked exhaust?

If I recall wide swings, vibrating Vacuum gauge needle was a sign of valve problems????

Ecomike said:

Just had a thought, I think the vacuum gauge readings were pretty steady at about 10-11 " of vacuum when the engine is miss firing, first at 3000 rpm, and now all the time at 300-500 rpm.

If my memory is correct, the fact that it was steady(?) and low, with good compression confirmed, would imply a bad Cat Converter, blocked exhaust?

If I recall wide swings, vibrating Vacuum gauge needle was a sign of valve problems????

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When you were able to rev the engine to 3000, did you let the throttle snap shut and observe a much higher vacuum?

As you might have guessed, I'm not a fan of aggressive chemical therapy. That much "stuff" going through the combustion chamber might have been the tipping point for the CAT.

Hypoid said:

When you were able to rev the engine to 3000, did you let the throttle snap shut and observe a much higher vacuum?

As you might have guessed, I'm not a fan of aggressive chemical therapy. That much "stuff" going through the combustion chamber might have been the tipping point for the CAT.

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Being a Chemical engineer, I am a fan, LOL. But you may be right, it may have dumped the loosed crud into the Cat and finished it off!!!

"When you were able to rev the engine to 3000, did you let the throttle snap shut and observe a much higher vacuum?"

Yes I did. There were no Vacuum gauge indications of any problems using all the Vac Gauge tests, except when it began bucking and popping out the exhaust during rapid acceleration or at high rpms like 3000, the Vacuum gauge did the same thing it does all the time now at 500 rpm, it falls to steady 10" of vacuum.

I did a lot Vacuum gauge diagnostic reading for 2 hours this morning, it can not be an intake blockage, that would increase the vacuum, and they all seem to say it can not be the valves, they show up differently on the Vac gauge tests.

So I am down to spark issues (plugs, wires, Cap, Rotor arcing or wire guts issues), bad dying CPS or bad Cat now I think?

Hell it could be the CPS and a bad Cat now. At this point I am glad I did not rush to pull the Head off.

http://what-when-how.com/automobile/engine-vacuum-test-automobile/

I am getting 4300 ohms on the cap to coil spark plug wire (that I used for the ground plug with a .060 gap test that 8Mud suggested), and 5700 ohms on the plug #3 secondary wire and 7300 ohms on plug #1 (it is longer than the short #3 plug wire). I think plug # 2 was about 5700 ohms also, I forgot the exact number. They all look new, barely 3000 miles on them. Wire #2 and #3 are the same length.

Did not test wires 4,5 and 6 yet. But I am moving on to tackle the Cat converter and CPS sensor after I move the jeep back on solid concrete today. And then I will try the Carb cleaner fuel feed only test before I tackle the CPS and CAT. I am convinced for now it is not a spark problem, which highly suggests a clogged Cat.

A low fuel trim (removing fuel) is likely a vacuum leak. With a large leak, the map sensor sees lower vacuum, which the computer interprets as load. It richens the mixture, then sees the exhaust go rich, and cuts fuel back. If you can't find any leaks with conventional methods, it may be time to take it to a shop and have them smoke test the intake and possibly have them do a cylinder transducer test. If the valves are sticking this test will prove it or anything else wrong mechanically.

Today's tests confirmed it is not the Cat Converter, Bummer.

On the other hand I have a new CPS, and the Damn Cat is welded in place anyway, can't drive it to a Muffler shop, I am not a welder. I dropped the exhaust pipe at the Exh Manifold to test the Cat. No changes. Sounded cool though. :eyes:

Also tried carb cleaner fluid feed into a WOT throttle body (intake hose removed), It just flooded it, died, then I dried it out, same thing. I sprayed some in, then cranked it, no difference, just longer to fire up. Throttle body is not the problem, fuel is not the problem, exhaust is not the problem.

Who wants to take bets that Old_Man is right way back 80 posts ago that the CPS is dying and is the actual problem?

Not me, not anymore, LOL.

Renix CPS gets swapped out tomorrow.

At this point I hope it is the CPS.

But so weird that it would act up for weeks a little at WOT, or near there, then at 3000 rpm, then aget a little worse at 2000 rpm, then overnight after I ran the carb cleaner through the intake, the CPS gets so bad it barley runs at 300-500 rpm? Co-incidence? Maybe.

When it ran still and had low STFT, I am sure it was at least in part the bad Vacuum line to the MAP. Its already fixed. The problem has been a moving target. There are no intake leaks anywhere, other than the one I found in the MAP feed.

Starting to look like a common HO CPS problem that is rare for the Renix CPS, but possible. Lets hope it is anyway. Will know tomorrow night.

trippled said:

A low fuel trim (removing fuel) is likely a vacuum leak. With a large leak, the map sensor sees lower vacuum, which the computer interprets as load. It richens the mixture, then sees the exhaust go rich, and cuts fuel back. If you can't find any leaks with conventional methods, it may be time to take it to a shop and have them smoke test the intake and possibly have them do a cylinder transducer test. If the valves are sticking this test will prove it or anything else wrong mechanically.

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Being as it is July 4th, and I have no fireworks, I am seriously considering tossing a grenade near the gas tank. But with my Luck it would be a dud. :laugh2:

So if I was not bored yet with the wild cloaked gremlin chase of the decade on the engine miss, I now have a new problem.:banghead:

How the hell do I get the second shoulder bolt safely out of the bell housing that it seems to have fallen into. :bawl:

I say seems, as this driveway has been known to eat bolts in a magical black hole that appears during vehicle maintenance. Its a magical place like Tahiti, LOL.

The last 2 were dropped into a Saturn Automatic transmission case 80,000 miles ago, and have yet to re-materialize.

So what are the safest step by step ways to retrieve it.

I am thinking manually turning the crankshaft clockwise at the front harmonic balancer. I am afraid to crank it, and possibly damage the Flex plate?

The CPS is pulled and out of the way, as is the cover guard that did come out with the old CPS, just with out the second Damn bolt. I was oh so careful too. Just not as nimble as I was 20 years ago.

Well the fireworks were cancelled.

I got the bolt, sheer luck, long story.

Had her running for 3 minutes at 3500 rpm today, let it idle and after about 10 seconds she died for good.

It was not the CPS!!!! New one was worse than the old one, had to do the Ecomike fix on it......

It was not the Dizzy needing indexing, but indexing it did lead me to the real problem.

It was not the valve train.

It was not the cap, the rotor, the plugs, the wires, the ICM, the HV coil, the ECU, the battery, the grounds, the owner :nono:, LOL, the blinker fluid,

It was not any of the usual suspects!!!!:laugh3:

The sheer pin holding the gear onto the dizzy shaft died, slowly, in stages, and today after running well at 3500 rpm for 3 minutes (after indexing the dizzy), it completely gave up for good, at idle after 5-6 seconds at idle.

Wasted a lot of time dorking with changing the CPS. The new one had 1/3 the peak AC voltage while cranking of the old one. But I was getting desperate and was not yet convinced it was bad valves on the head. I fixed the new CPS with a hammer and a long crow bar,:twak: literally :laugh3: LOL. Got the cranking voltage up from .20 (even after drilling out the holes to one size larger and slotting the bottoms side), to .65 just by bending the arm. I think pushing it away from the gear teeth a little bit may be the real trick on Renix CPS performance boosting?

Putting the new dizzy in Wednesday when/if I get time.

That was a tough one, glad you finally found the problem.
When a shear pin fails like that, it makes for very hard
diagonosis. Must have been a binding distributor bushing
to make the pin fail...?

Anyway, super job on finding it.

Well she is not through playing games yet.

But she is a runnin again, LOL:laugh:

So I reconnected the MAP sensor (ran pretty good with out it, kinda shocking :eyes:, LOL) that solved the open loop problem, cleared the Renix KAM memory, and all the Snap-on MT-2500 data on all the sensors is now normal and in closed loop (MT-2500 jumped up, slapped me and said hey fool, where is MAP sensor vacuum line, LOL)....so now all the sensors and data look awesome. Short term fuel trim is tending towards a rich correction, running from 60-85 or so, for starters, but way better than before.

But it now has a slightly rough idle, very stable idle speed, but not supper smooth, and give it a little gas and it shakes some till you get past about 1300 rpm. Smooth as glass at 1500 to 2800 rpm.

Still wants to miss and backfire at about 3000 rpm, even if I slowly creep up on that speed. Also seems to want to miss and back fire If I stand on the gas pedal at about 1200 rpm, but I need to really goose it.

Would the brand new, but carbon fouled spark plugs cause an idle miss?
The rough idle (constant speed) is new. Before the idle was smooth and the miss was only at 3500 rpm when it first started. Then it got worse over time, but I now know that part of the worsening problem at some point was the leaking MAP vacuum line, and then later the sheered sheer pin that holds the helical gear on the Dizzy drive shaftthat got so bad it would barely run and only ran at about 300 to 500 rpm. Could not get it over 500 RPM even at WOT!!!!

The fact it runs as smooth as glass now at 1300 to 2800 rpm seems to indicate a good head and valves, but the sudden miss, power drop and severe back fire as I sneak up on 3000 rpm has me thinking valves?

What do you all think and this point?

I have not indexed the brand new distributor (cutting off the location ears and doing the TSB-Jeep-Cruiser54 suggested MOD), the Dizzy is brand new, and the rotor tip is pointing exactly where mine pointed for 13 years and 70,000 miles before with no miss or backfire issues before.

Oh, the old dizzy also had a very loose sensor inside the dizzy!!!!

I wonder if all the back fires during diagnostic testing finished off the sheer pin. Also 13 years and using thick oil that starts the rig up at 55 psi with an aftermarket high flow oil pump, and the fact it was a rebuild Motor craft dizzy, its possible the sheer pin was 30 years old and had prior damage before I got it, all may have been additive to cause its failure?

The question is what do I try next?

I suspect the head, but after finding the MAP sensor Vac line issue and the Failing, and finally failed Dizzy shear pin, I am not so sure.

Should I try a set of remanufactured, matched flow tested injectors?

The only remaining sign of any issue with the MT-2500 scanner is a low STFT, at idle so far. I am reluctant to cut up a new Dizzy that is under a lifetime warranty to do the TSB indexing. Like I said the old dizzy, and the one on my other 2 running Renix rigs are not indexed and hacked, and they point dead on at the #1 pin at TDC with no problems like this new one does. I will check all three one more time.

Can a valve train problem cause my problems???

Runnin'OnEmpty said:

That was a tough one, glad you finally found the problem.
When a shear pin fails like that, it makes for very hard
diagonosis. Must have been a binding distributor bushing
to make the pin fail...?

Anyway, super job on finding it.

Click to expand...

Index the dizzy.

Thanks. What were the Factory TSB problems the Dizzy indexing TSB was said to fix?

Why did this same engine, and my other 2 Renix jeeps work perfectly with the rotor pointing directly at the #1 pin on the cap for 13 years, 290,000 miles, 11 years, 245,000 miles, and the third.... ... with out any of them ever needing indexing?

cruiser54 said:

Index the dizzy.

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cruiser54 said:

http://cruiser54.com/?p=68

Click to expand...

:worship:

cruiser54 said:

http://cruiser54.com/?p=68

Click to expand...

Done. New Dizzy, index according Cruiser54's how to guide.

I now have a rough idle. Idle speed is still rock steady, no speed wander, Running closed loop, STFT is running 60-85, max range. I see about 1-2 lbs of very rapid vibration in the fuel pressure gauge at idle. I am thinking vacuum is changing at the FPR, as the fuel pressure vibration goes away at 1500-4500 rpm, and so does the engine shake. But I need to verify it is vacuum and not the FPR itself.

Engine smooths out 1500 rpm and operates perfectly from 1500 to 4500 rpm. So it is not fuel pump.....or HV coil, or ICM or CPS (all of which were replaced and or swapped). Plugs have under 30 minutes of run time.

If I gun the gas pedal and then go back to idle on the gas pedal, between 1500 and 4500 rpm there is no back fire or stumble now. Runs great from 1500 to 4500 rpm now in park.

But it shakes some at idle up to 1500 rpm, and if I stomp on the gas at idle I was able to get a small back fire out the intake once. Did not want to press my luck. The idle was perfect before all this started, and the back firing and miss, and power loss was at 3500 rpm at first, then at 2500 and then finally at 300-500 just as the Dizzy sheer pin started to break and let go for good.

The idle slowed down some as well since the NEW dizzy was indexed, running about 650 rpm cold, maybe 700 rpm warmed up, Was at 750 rpm before the dizzy was indexed IIRC, and before the rough idle started.

But the idle was perfectly normal before the old dizzy sheer pin went bad, which was after hours of trouble shooting the high rpm back firing problem that started this thread madness, LOL.

At least now it runs and get running data. So far the only thing that shows a possible issue is the STFT still under 128, running 60-85, but way WAY better than the 30-0 I was getting. Goes right into closed loop super fast and stays there.

Anyone have any experience with spraying water into the intake to remove carbon in the combustion cylinders? I know it has lots of carbon, I could see it on the top of piston #1 while setting TDC, and it was running way rich for hours while debugging it. Can't help but think carbon may be the idle problem, but then again it may not be.

So what could be causing the new rough idle? Slightly, occasionally leaking intake valve at low rpms (does that even make any sense?)? I think the vibrating fuel pressure at idle, the shake at idle, the back fire out the intake gunning it at idle (no more backfires out the exhaust at all now!!!).

I will report back after running a new series of vacuum gauge tests on the intake. Yesterday the vacuum looked good, but the idle vacuum was 15" cold and 16-17" warmed up to 195 F (increased as the engine warmed up).

Vacuum looked great during accel/decel tests before I fine tuned the dizzy. I had eyeballed it last night, but today with a cut out cap and a run at getting to TDC #1 again, I needed to index it about 1/4" more today. Had the rough idle at both settings, and the factory OEM setting before cutting the tabs had the rough idle.

Indexing the dizzy did eliminate the the higher rpm and gunning it back fires from 1500 to 4500 rpm. Thanks Cruiser54!

I've waterboarded many a Jeep 4.0.

Do it.

Same here, get the engine warmed up first.

LMAO. :roflmao:

cruiser54 said:

I've waterboarded many a Jeep 4.0.

Do it.

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