Engine miss above 2000 RPM

[Ecomike]

87 Renix rig, 292,000 miles.

I have had worn valve guides and oil consumption and fouled plugs I worked around for 70,000 miles that have gotten worse, and stick lifters that stop sticking if I ran the RPMs up in cold morning starts to unstick them, and when the #1 and #2 plugs would foul I would get miss like I have now that was always fixed with new plugs or by cleaning the old plugs of the oil fouling.

But now the problem is different, and the miss when I accelerate past 2000 rpm (varies, when hot it will go to higher rpms with out the miss, some times) not only misses but back fires out the exhaust. Pop pop pop sounds and power stalls and rpm stops rising as you give it gas.

I ran the MT-2500 Snap on sensor tests, ran a vacuum gauge test, ran the fuel pressure tests, Sensors are all good and working, TPS is 3 months old Mopar, CPS is firing like a demon, and I ran the compression test and my worst two cylinders are actually better after using Restore for 4 years and the NGK FR5 plugs and fixing the fuel injector issue...and the idle is perfect

But the compression test does not tell me if a valve is misaligned and not seating under acceleration or at higher pms, as it does at cranking speed.

I also used a 25 year old Matco engine systems analyzer I scored on Ebay 10 years ago that gave me Ms rise times and peak and average HV firing data that indicates that the spark, and power contribution per cylinder all seems to be OK, even excellent as well. Getting close to 17 mpg pure highway at 60 mph, O2 sensor is 12 weeks old working perfectly.

None of these tools gave me any clues to what is causing the miss-back fire even when I watched them while I made it back fire and idle badly for 10 seconds, they all indicated no problems, but when the problem happens I can not accelerate in park to higher RPMs with out it back firing and shaking.

IF I let her idle for 20 minutes (get warmed up), or just sit for 2-3 days, or wait for a full moon the problem is not as bad, sometimes, and one day it may just refuse to go over 2000 rpm in park before it shakes violently, the next day it can reach 4000 rpm for a bit, and in between.

The rapid acceleration miss showed up at first for a few seconds on cold morning starts but would go away 100% if ran it at 4000 for about 10-30 seconds, then no problems all day. A slow climb in rpms is less likely to trigger the miss, but on other recent tests it just bogs down at 2000 to 2200 rpm and refuses to go higher, day later I warm it up 20 minutes then I managed to get to 4000 rpm for a 10 second stretch, before it barks at me

5 days ago the problem decided to stay for good, till today. Today it was still there, but not near as bad. Bad enough though that I have parked it.

I do plan to pull the valve cover next.

Looking for ideas on what I might have missed.

Coolant and oil are normal. No over heating. Tests and problems are in park!!!!!

The vacuum gauge showed no compression or valve issues at all even when I made it miss-backfire, which kinda stumps me.

Fuel pressure was stable during the miss.

Plugs 1 and 2 were replaced, made no difference. Dizzy rotor and cap have only 2000 miles on them and are fine. No vacuum leaks, idles like a champ!!!!

I plan to reconnect the new Knock sensor and see if anything changes next. I doubt it will, but worth a try as I did hearing some pinging on acceleration off and on the last 2-3 months if I tried to push her in low gear too hard.

This feels mechanical like valves/valve-lifter train on the edge of going bad, but I am no expert on those in failure mode.

Oh, EGR passed the tests, not the problem. Plugs and wires all great, not much age on them.

I'd suspect the ICM-HV coil if not for having run the Matco analyzer tests and them indicating no spark problem during the steady misfires.

 

[old_man]

Sounds like the engine is carbon'd up. Run a can of BG44k or Seafoam it.

You could pull the plugs and take a look. Use a bore scope to look inside the combustion chamber and on the back of the valves.

 

[Ecomike]

I have never seen carbon that bad, maybe you have?

It is not and has not run rich in 70,000 miles since I bought it 12 years ago, why would it be carboned up now? The plugs look great, no carbon or oil fouling, O2 sensor and other sensors are have been good, running at stoich, switching lean-rich properly and timely for a along time.

Sounds like the engine is carbon'd up. Run a can of BG44k or Seafoam it.

You could pull the plugs and take a look. Use a bore scope to look inside the combustion chamber and on the back of the valves.

 

[old_man]

I understand your frustration. I had a similar problem and it turned out to be a bad CPS. Every thing was fine until you hit about 2k rpm and then all hell would break loose. It had me scratching my head for a while.

If nothing else, hook up a timing light and watch it while you increase the rpms. Two things that tend to be rpm related are the coil/ignition module and the cps.

 

[Ecomike]

Was that a Renix CPS or an HO CPS that had that problem? I seem to recall the HO CPS it is a common issue at 2000 rpm miss firing, but not sure the Renix CPS fails that way? Mine has maybe 10,000 miles on it, if that. Bad Renix CPS normal causes a hard to start situation and mine fires up nice and fast as it has the Cruizer54 mod on it. I will check and clean the connector pins!!!

If it was the CPS I might be able to detect it with my analog volt meter.

But the Matco Engine analyzer I dug out and used

MD59 also listed as a Ferret Instruments 59 Direct Ignition Analyzer (1994) One of the first laptop sized engine analyzers of the day.
(PM an email and I will scan and email the manual if you want read it, its right up your EE ally)

showed no changes in the max or min firing voltage, or in the power contribution per cylinder even during the constant stumble miss fire at >2000 RPM levels. I would think it would have shown data that the manual says it should show that would indicate a plug-plug wire-ignition coil firing late or early issue??? I did not use the injector test with it yet.

What about a sticking fuel injector? Today it back fired once and I could smell gas 2-3 seconds later at the drivers seat???? I have not pulled that fuel rail in 70,000 miles, 10 years that I recall since I changed the leaking body OEM injectors.

That has me thinking injector or valve. I may pull the fuel rail first, but why would and injector stick only at 2000 rpm and higher????:banghead:

I strongly suspect the rocker(s) is side loading on a valve again (the cause of the worn valve guides I need to deal with sooner or later) and not sealing all the time at the proper time, but I have never had this exact problem in my 45 years, so wanting to leave no stone unturned.

I have a timing light, but what will it tell me, and when? I am already surprised the miss fire did not give me any bad tester data of any kind. What the symptoms of a valve problem? Could be a stuck lifter finally be going completely south at higher rpms?

One last thing, and this has me thinking, the miss yesterday was way worse in park, and almost nonexistent in drive around 2000-2500 rpm. What difference would a load on the engine make????? Not sure if that was a fluke, or repeatable yet.

I understand your frustration. I had a similar problem and it turned out to be a bad CPS. Every thing was fine until you hit about 2k rpm and then all hell would break loose. It had me scratching my head for a while.

If nothing else, hook up a timing light and watch it while you increase the rpms. Two things that tend to be rpm related are the coil/ignition module and the cps.

 

[Hypoid]

Are these the original injectors?

 

[Ecomike]

Are these the original injectors?

I don't recall for sure if I replaced all 6 on this rig or the other one.

I know some were originals leaking at the body seam and those got replaced with brand new $60, 4-hole yellow body injectors from Five-O online. I think some may still be what was on it when I bought it at 224,000 miles about 10-11 years ago, so I have no info on them, but I have had zero injector problems since 2007, no leakers like the OEM injectors, about 60,000 miles on this rig/injectors with no problems, and it idles as smooth as glass. I would think an injector problem would show up at idle? But I am not an injector fault detective :laugh3: expert, LOL.

 

[cruiser54]

Have you done this?

http://cruiser54.com/?p=41

Or substituted a know good assembly?

 

[old_man]

It was a Renix that had the weird CPS.

 

[Ecomike]

It was a Renix that had the weird CPS.

I wonder if a volt meter on Low AC setting, digital would catch the signal dropping out at 2000 rpm. But if I am using the Matco Engine analyzer properly, it is designed to see and report that problem. Also the Snap-On MT-2500 has showed no sensor or other problems yet either, making me think it must be a valve-lifter issue.

 

[Ecomike]

I forget why, but I did swap the good working one from the rolled jeep in there about 3 months ago. But I do not recall why I did that. Both of them were working properly, and cleaned up well enough to eat off of, LOL.

And the Matco engine analyzer has emf pick ups that sense the amps, KV and rise time of the secondary and primary coil sires and the power signal going into the ICM that charges the HV coil, and they all were normal even during the miss. I might test that theory by leaving a spark plug boot loose and see if it shows me problem.

I think I either have too much fuel, late spark or a leaking exhaust valve when the > 2000 RPM problem happens. That is what my limited experience and gut tell me, but.....the problem is 100% non existent under 2000 rpm

Have you done this?

http://cruiser54.com/?p=41

Or substituted a know good assembly?

 

[cruiser54]

still have a cat on it?

 

[Ecomike]

still have a cat on it?

Yes but the exhaust flow feels normal for this engine, Cat was changed with the O2 sensor back a ways, and the problem is during rapid acceleration even at 1500 rpm now, on a cold start. I will recheck it. The Cat is noise free, and I have had it up 4,000 rpms if I do it slowly and when its good and hot.

I am starting to wonder if I have my first MAP sensor failure, going to retest that and reconnect the Knock sensor

I disconnected it and test swapped and swapped again and cleaned the ICM-HV coil 3 months ago when I was tracking down the source of death wobble, that was a bad tire-wheel, and steering parts that I finally fixed. At first I thought it might be the engine.

Why would it be way worse when the engine is not under load in park when I goose the gas pedal? Because the rpm changes slower under load and the ECU has more time to respond??????

 

[cruiser54]

Cruiser’s Vacuum Test for Exhaust Restriction

Your vacuum gauge should come with an instruction booklet outlining this procedure.

Hook the vacuum gauge up to a source on the intake manifold.
Start the engine and note the vacuum reading.
It’s usually 17 to 21 inches of vacuum.

Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle.

Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading.

If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.

 

[Ecomike]

Thanks, it already passed that test. All of it. I had forgotten that it can confirm a blocked exhaust :clap:. Also there was no vibration or rapidly oscillating needle.Looked completely normal even while shaking at a fixed 2500 rpm or while goosing it and making it shake.

It may be time to rerun the tests. And run the gumout through it.

I also want to reconnect the Knock sensor I disconnected when it had death wobble to see if this some kind preignition that the knock sensor can stop, but it suspect it is late, or rich ignition out the exhaust when it is shacking.

Keep the ideas coming folks, please.

Cruiser’s Vacuum Test for Exhaust Restriction

Your vacuum gauge should come with an instruction booklet outlining this procedure.

Hook the vacuum gauge up to a source on the intake manifold.
Start the engine and note the vacuum reading.
It’s usually 17 to 21 inches of vacuum.

Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle.

Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading.

If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.

 

[Ecomike]

Fresh data.

The MAP is good. It ran better when hot tonight.

The O2 sensor shows that is falling out of closed loop randomly!!!

the O2 sensor is only about 4 months old and it goes into closed loop quickly at idle after a fresh start, and stays there at idle till I play with the rpms and punch it. Then some times it stays in closed loop others not, no pattern too it but it did get stuck reading LEAN at .68 volts (0-5 V is the Renix O2 sensor range) for a while after a back fire at about 3000 rpm, til I ran the rpms up and down again and got it back the closed loop.

Also the Short term fuel trim STFT would drop to 0 some times and hover there for a while and sometimes get to 128, LTFT was 128 steady.

So with the good, no problem found, prior fuel pressure gauge tests (I will re run and confirm) it looks like fuel injector- and or wiring etc fuel rate issues...., but not sure I have ruled out sticky valves, or even poor spark for 100% sure yet.

I do not think the O2 sensor is bad.

Can brake fluid leaked on the sensor foul it up??? I forget? We did brake work last week and I may not have washed it off properly, I forget. But since it works fine till I create a miss, and sometimes recovers, inclined to think it is not the cause but smoking gun confirming a possible fouled or sticking injector issue?

THoughts?

Cruiser’s Vacuum Test for Exhaust Restriction

Your vacuum gauge should come with an instruction booklet outlining this procedure.

Hook the vacuum gauge up to a source on the intake manifold.
Start the engine and note the vacuum reading.
It’s usually 17 to 21 inches of vacuum.

Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle.

Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading.

If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.

 

[Hypoid]

So with the good, no problem found, prior fuel pressure gauge tests (I will re run and confirm) it looks like fuel injector- and or wiring etc fuel rate issues...., but not sure I have ruled out sticky valves, or even poor spark for 100% sure yet.

I do not think the O2 sensor is bad.

Can brake fluid leaked on the sensor foul it up??? I forget? We did brake work last week and I may not have washed it off properly, I forget. But since it works fine till I create a miss, and sometimes recovers, inclined to think it is not the cause but smoking gun confirming a possible fouled or sticking injector issue?

THoughts?

I was hoping that Cruiser54 nailed the problem with his tip on cleaning the coil primary contacts. I did help a friend isolate a bad ignition module that tested good on the bench, after it had cooled off.

My first thought was a bad injector or poor electrical connection causing the injector to not operate.

My second thought was an un-burned charge lighting off in the CAT.

My third thought is an un-discovered fault in the secondary side of the ignition system.

Have you tried pulling plug wires while it is misfiring?

 

[Ecomike]

Well here is what I know and fresh data from the engine for 30 minutes in park.

FPR and fuel pump are working perfectly all the time.

Spark coil, ICM and CPS working perfectly all the time.

Runs better once it is good and hot, or at least it did today LOL.

Only drops out of closed loop when I rev the engine hard, acceleration or push the rpm up till it misses, then it switches to open loop and stays lean for up 1-2 minutes at idle if I make it miss for a good 5-10 seconds and then it goes back to closed loop.

Primary spark test was perfect text book data at idle. KV, Build time, burn time all seemed normal. The secondary spark data was also OK on the averages, at idle.

STFT Short Term Fuel Trim is sometimes 128 (normal), but more often than not, most of the time it goes from 20-50 and even as low as 0-? at IDLE!!!! same at all RPMs, IIRC. So that tells us something, but what?? Could not get any idea what was causing it, yet.

Did see it go to 128 STFT at idle in closed loop late in tests.

Spark KV passed ALL tests, all of them. 8-10 KV at idle, 19-20 KV when quickly revved up. Book says that is good.

So when it goes to open loop it reads way lean.
STFT goes way low

The last tests were looking at the min and max burn times on each of 6 cylinders. It is telling me something for sure!!! They looked OK at idle

But the Min and Max burn times were way off when the engine was revved ( and 2/4 cylinders were off in 2 different directions), and it looks like I can see a huge difference in the burn time from the two FR5 spark plugs and the four single platinum tip plugs I have used for 12 years on all my jeep 4.0s with no problems. I switched to the two FR5 NGK plugs on Cyl #1 and #2, as they are hotter and do not wet foul with oil from the leaky worn valve guides on those two cylinders. But I did that 18 months and about 4000 miles ago with no problems till last week.

The lean reading in open loop, which should be rich in open loop, and the STFT issues has me suspecting an issue at the fuel injectors, as misfires from a plug issue should make the engine run rich, not lean.

The fuel injectors are all brand new Five-O-motorsports 4 hole injectors installed abut 60 miles ago, 12 years ago.

I am wondering if the 2 injectors on the two cylinders using the FR5 plugs are fouled with burned oil, as I have had to pull and clean the plugs 3 times on those two.

Can injectors get fouled/sticky such that they work properly at idle but starve the cyl for fuel at higher load?

I have early notes, that I need to recheck that indicated a high burn time, the book says can be caused by low compression (well it does have 290,000 miles on it, but the compression is not below the 120 Psi spec, in fact it is about 140-150 ), other causes it listed rich fuel mix (ruling that out as the O2 sensor does work and got the high burn times, averages, at idle, spec is 1-2, and I got 1.9-3.0, but max burn times can go way higher when revving the engine, so not sure I read too much into the idle max burn times, and a later test of the secondary burn times at idle was 1.6-1.8, which was awesome!!!! The other possible cause of long burn times was fouled spark plug gaps, and two of the plugs are brand new and the other four went 50,000 mile and looked brand new still so that is out.

Last test I can run with the Ferret engine analyzer is the fuel injector build and drive time test of each injector, but I suspect that is not affected by plug tip fouling that screws up the spray pattern, or chokes back the flow rate.

BTW, I love this old tech engine analyzer. It is awesome, now that I know how to use it. It will do stuff an OBD-II and Snap-on MT-2500 can not do!!!It can show a stuck or wiring partial fuel injector coil short fault in the injector itself, or a temporary loss of signal to an injector. So I may that test next just to be sure. It uses a pick up coil to sense the time domain and the voltage and current passing through the HV wires and feeds to the ICM and fuel injectors. A lost art and tech nowadays??? It was built in 1994. I scored it on Ebay about 11 years ago, dirt cheap. I still looks like it was never used.

But I am thinking of pulling the fuel rail next. I have not looked at the valve train recently. But today's tests and issues were with a quite motor with no lifter or tappet noise.

What am I missing folks????

Vacuum gauge, Fuel pressure gauge, volt-ohm meter, Snap-on MT-2500, all indicate that all else is normal

Down to the difference in the spark plugs,

the fuel injectors maybe being fouled (or ECU, or wiring, which I doubt to be the problem),

STFT is not normal - Why??

Got her up to 3000 rpm with little trouble today, had a harder time making her miss, but while it would miss at 1200 rpm at cool start up when revving 2 days ago, now I must push it up to 3000 or rev it real hard up to 2500-3000 from idle, with it very well warmed up today.

It slips back into a very lean open loop state when I make it rev and misfire. Therefore I suspect the misfire is a lean fuel misfire????

I was hoping that Cruiser54 nailed the problem with his tip on cleaning the coil primary contacts. I did help a friend isolate a bad ignition module that tested good on the bench, after it had cooled off.

My first thought was a bad injector or poor electrical connection causing the injector to not operate.

My second thought was an un-burned charge lighting off in the CAT.

My third thought is an un-discovered fault in the secondary side of the ignition system.

Have you tried pulling plug wires while it is misfiring?

 

[cruiser54]

What is your fuel pressure for 30 minutes after shutting it off?

 

[Ecomike]

What is your fuel pressure for 30 minutes after shutting it off?

I think it was 25 psi. I need to go check right now I guess.

Edit: It is 21 right now, been about 30-60 minutes I think since I ran it again

and ran injector tests and confirmed that cyl #1 and #2 fuel injectors are operating properly electrically, on three power and time tests using the
Ferret engine analyzer. That does not test if they are sealing properly.