Engine My turn for a Sputtering-hesitating issue

Chythar

Recently finished repairing my rear
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Issue happens at all RPM's, and starts up within a few seconds of starting the car. Engine RPM's go up and down maybe 200 RPM each time, and drop low enough at idle that the engine threatens to die. If I leave it long enough, the engine will die like I turned the key. This is the weird part - the RPM's don't slowly drop. It;'s not a surging idle that goes up and down, like you tapped the gas pedal and let the RPM's drop down. The RPM's drop almost instantly like the whole engine has lost electrical power. Less than a second later, the engine recovers and the RPM's jump back up almost instantly. Engine revs like normal, but higher RPM's don't seem to affect the issue. The engine doesn't sound like a miss on a cylinder or two, it sounds like the whole engine is cutting out. I can record a video on my phone if the description isn't clear.

I first thought it was a fuel problem, so I replaced the fuel filter. No change, though it was at least 10 years old so it needed to be done.

Next, I put a fuel pressure gauge on the schrader valve on the fuel rail, then started the engine. I had a constant 38 psi as the RPM's bounced up and down. I pulled the vacuum hose off the fuel pressure regulator, just to see if anything changed. The fuel pressure went up a couple of psi, but was solid until the engine died. Fuel pressure regulator was replaced in 2019, but the steady fuel pressure tells me it's not a fuel problem.

I don't know electrical very well, but I did put a voltmeter across the battery. It was 11.88v with the engine off, and fluctuated between 14.38v and 14.44v while the engine ran. I couldn't tell if the tiny fluctuations happened when the RPM's jumped, but as far as I know anything over 14v is fine.

As I said, I don't really know engine electrical or how to test it. My first guess would be to check if engine electrical was fluctuating with the RPM's, but again not sure how to test it. I'm very willing to try and learn, I'm just not sure what to do.
 
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General karthief

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Hook your dvm to the wire on the alternator that runs to the solenoid or hook it to the battery to see charging volts, the 11.8 volts static would concern me a little.
 

jozsefsz

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By your description, that's what I'd always assumed surging idle was when applied to the SN95. :) On the low-side, it's a drop-off in RPM as if the car were shut off, and sometimes a complete stall, and then it'll catch itself and rev back up. It's usually just the IAC flipping back and forth with some delay between being too far closed and then too far open. I think if you made adjustments (throttle blade and idle air bleed) to have the car barely run with the MAF disconnected, you'd be in better shape. If it's something a little different, maybe that video would help.
 

Road_Runna

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Issue happens at all RPM's, and starts up within a few seconds of starting the car. Engine RPM's go up and down maybe 200 RPM each time, and drop low enough at idle that the engine threatens to die. If I leave it long enough, the engine will die like I turned the key. This is the weird part - the RPM's don't slowly drop. It;'s not a surging idle that goes up and down, like you tapped the gas pedal and let the RPM's drop down. The RPM's drop almost instantly like the whole engine has lost electrical power. Less than a second later, the engine recovers and the RPM's jump back up almost instantly. Engine revs like normal, but higher RPM's don't seem to affect the issue. The engine doesn't sound like a miss on a cylinder or two, it sounds like the whole engine is cutting out. I can record a video on my phone if the description isn't clear.

I first thought it was a fuel problem, so I replaced the fuel filter. No change, though it was at least 10 years old so it needed to be done.

Next, I put a fuel pressure gauge on the schrader valve on the fuel rail, then started the engine. I had a constant 38 psi as the RPM's bounced up and down. I pulled the vacuum hose off the fuel pressure regulator, just to see if anything changed. The fuel pressure went up a couple of psi, but was solid until the engine died. Fuel pressure regulator was replaced in 2019, but the steady fuel pressure tells me it's not a fuel problem.

I don't know electrical very well, but I did put a voltmeter across the battery. It was 11.88v with the engine off, and fluctuated between 14.38v and 14.44v while the engine ran. I couldn't tell if the tiny fluctuations happened when the RPM's jumped, but as far as I know anything over 14v is fine.

As I said, I don't really know engine electrical or how to test it. My first guess would be to check if engine electrical was fluctuating with the RPM's, but again not sure how to test it. I'm very willing to try and learn, I'm just not sure what to do.
14's on the battery while engine is running? Dear lord, I'm in need for a new alternator.
 

Chythar

Recently finished repairing my rear
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As always, life gets in the way. But today, I was able to do some checking. My first check was the CCRM. I couldn't get to the wires I wanted to test with the CCRM bolted in, so I unbolted it and was able to pull it and the harness up higher. That's when I noticed the plastic connector on the CCRM was loose. Not that the plug on the wiring harness wasn't plugged into the CCRM all the way, but the light gray connector that's on the CCRM moved separately from the black CCRM body.

Voltage in and out of the PCM relay at KOEO was fine, so I started the car. And it idled fine. WTF? It didn't cut out like it did before. it ran and idled fine for a bit, then the idle dropped and almost died. I opened the throttle from the engine bay, and the engine revved perfectly fine - no stuttering or drop in RPM's. Double WTF.

I tried wiggling the CCRM connector while the engine was running, but I couldn't tell if the wiggle was causing the engine to die or if it was a coincidence. But the CCRM connector shouldn't wiggle like that, so I'm replacing it with one from LMR. A used one would be cheaper, but I'd rather have one that's been rebuilt and IN THEORY has been tested.

The LMR CCRM should arrive on Friday, so I'll report back with my results. With cars as old as ours, this may be just one of several failed parts.
 
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Chythar

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Put the new CCRM in today, and the issue is still present. And once again, the idle changed. It's gone back to the idle jumping down like it did when I first posted, but not quite as bad. The only thing I have done is replace the CCRM. Here's a video to show the issue. The engine started slow because the battery is low, I can't run the engine long enough to charge the battery. I've been using a battery charger to top it off.

 

jozsefsz

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From the tach that 100% looks like the standard SN95 surging idle, mine started doing exactly that about 1997 and gave me fits until I found jrichker's famous checklist. IAC closes, throttle plates are too far closed not letting any air in, engine threatens to die or dies, IAC panics and opens wider and idle shoots up. Unplug IAC, tinker with the idle stop screw or idle air bleed until car stays running around 600rpm. Most of the time no TPS resetting needed on an SN95. The surging idle checklist should do the trick, if you're confident you have no vacuum leaks or electrical issues (the surging indicates the IAC is actually working - just too far delayed as it's really meant to handle major idle load situations like turning on the A/C not to fine-tune your idle) you can skip to post #2 setting the base idle speed. The throttle stop screw wears down over the years, dirt accumulates on the throttle blade and in the IAC, and the throttle blade just closes too far. This causes the choking and gasping you're seeing. No matter what, it's free to try and won't take you but 20 minutes tops, and it won't hurt anything.
 
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Chythar

Recently finished repairing my rear
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From the tach that 100% looks like the standard SN95 surging idle, mine started doing exactly that about 1997 and gave me fits until I found jrichker's famous checklist. IAC closes, throttle plates are too far closed not letting any air in, engine threatens to die or dies, IAC panics and opens wider and idle shoots up. Unplug IAC, tinker with the idle stop screw or idle air bleed until car stays running around 600rpm. Most of the time no TPS resetting needed on an SN95. The surging idle checklist should do the trick, if you're confident you have no vacuum leaks or electrical issues (the surging indicates the IAC is actually working - just too far delayed as it's really meant to handle major idle load situations like turning on the A/C not to fine-tune your idle) you can skip to post #2 setting the base idle speed. The throttle stop screw wears down over the years, dirt accumulates on the throttle blade and in the IAC, and the throttle blade just closes too far. This causes the choking and gasping you're seeing. No matter what, it's free to try and won't take you but 20 minutes tops, and it won't hurt anything.
True enough. I did unplug the IAC to see if it had any effect, and the RPM's simply dropped until the engine died. Which sounds like exactly what one would expect with a surging idle. I'll go through the checklist and see what happens.
 
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Noobz347

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Sneak some wire in there so that you can get a 12-volt reference from the coil input when this stutter takes place.

The lack of any popping through the exhaust when this happens leads me to believe that you do have an electrical issue. The alternative would be that something is causing the fuel injectors to completely shut off (nothing comes to mind as to why this might happen).
 

Chythar

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Didn't get too far with the checklist 'cause my cooling fan didn't turn on during KOEO. Got a 564 code for it. Only stored code was 212 for the PIP, but it went away the next time I ran KOEO, and I didn't clear codes. I wiggled the fan connector to make sure it was in tight, and ran KOEO again. Same 564, but now I have 553 - Secondary Air Injection Diverter circuit failure. It's been a while since I've ran a KOEO, but doesn't the test usually turn on and off a number of relays and solenoids, making several audible clicks? I presume the test stopped when the fan didn't turn on.

The really strange part is that I got a deep bass hum out of my radio speakers while the KOEO test was running, It happened a few times in the middle, then one loud hum at the end. Never had that happen during a test before.

The cooling fan issue reminds me of a problem I had when I first did the Cobra engine swap, so I have wiring to check. But I also want to see if this issue sparks some other ideas amongst you fine motorheads.
 

Chythar

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Checked what I hoped was the problem wiring, but it seems to be fine. When I did the Cobra conversion, the donor Cobra's fuse box was crushed in the accident that bent the unibody. I ended up re-using my V6 fuse box, but I had to cut and splice in a connector from the Cobra harness. If I remember right, it was because my V6 did not have ABS and Cobras do. Anyway, I had a hard time soldering these wires and a sharp edge ended up causing a short. The only symptom I was aware of was that my cooling fan did not come on. I was guessing the current issue was related, but it doesn't appear to be. I unwrapped the wiring harness loom, and the improved soldering & wrap on the wires is intact. The four solder joints are on the left side.

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I don't see any cut insulation on the wires, or any cause for a short. Though I think I'm on the right track. I had a weird issue come up back in June, where my right headlight and turn signal were dim while driving. Some time later, the headlight and corner light suddenly went back to their full brightness. The wiring for those lights run along this part of the harness.

I'm going to follow this part of the harness back to the fuse box, and see if I can find a short somewhere.
 

Chythar

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Still poking around. To recap, I started the Surging Idle checklist but got stopped at the KOEO test because my cooling fan wouldn't turn on. Only code was 564 for the fan, no stored codes. I have verified that the low-speed fan works by hooking up wires directly between the connector on the fan and the battery. The fan connector on the wiring harness isn't melted. I've checked the fan's 60A fuse in the under hood fuse box, and it's good. I have a re-manufactured CCRM that I bought from LMR that is not known good, so I may drop in the old CCRM and see if the fan works.

I've got some other electrical gremlins beyond the engine surging idle, namely the right headlight and corner light going dim for a while but returning to their normal brightness while driving. This tells me I have an electrical short somewhere. It might not be related to the fan and engine issues, but it needs to be fixed. I had to splice and fix some wires when I did the Cobra conversion; one of those splices caused a short back then and my cooling fan wouldn't turn on (engine almost overheated). I repaired that splice back then; checked that splice and it's still good. I've pulled apart all the connectors between the CCRM and the under hood fuse box, and nothing looks charred or corroded. The wire loom isn't worn through either. I've been looking for cut or cracked insulation on the wires that might cause a short, but I haven't seen any yet.

Short of stripping the wire loom off the entire harness, anyone have any ideas of what I can check next? Anything obvious I should check?
 

WhiteCobra95

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Try shaking the harness in different spots with the headlights on. It sounds like that issue is intermittent, which is the worst kind of wiring problem to isolate. If you see the brightness of the light changing while shaking a specific part of the harness, you may be able to isolate the location of a short or open circuit. Start of small and controlled and make sure that when you way the loom it doesn't transmit way downstream of the location your rattling. Look for areas where the harness is loose, pressed, or could have rubbed through. There shouldn't be any large spans of un-secured harness, so if you find one, check what's connected at both ends. (Unsecured stretches of wiring can result in high amplitude movement of the harness which transmits into terminal movement. Over time the terminals moving can etch the pins they're connected to and build up a high resistance barrier.)
 

WhiteCobra95

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On the floating idle: do you have a way to log ECU data? I would be interested to see what the sensor signals are doing. It definitely sounds like the surging idle issue / throttle stop fix / IAC authority that jozsefsz mentioned, but if you have some strange shorts or grounding issues your sensors could also be floating. This should be easy to spot if something like your TPS is moving without a pedal input or your coolant or IAT is changing dramatically.