850T - Intermittent stumble/very rough running

[Roger_850T]

I'm having trouble with an intermittent stumble on my '95 854T. It happened today, although it's happened before, too. Today, I ran for 15 miles at 70mph on the interstate. As I rolled up the exit ramp I could hear the engine start to run rough. As I pulled up to the light, it ran rough and stumbled. It would not rev, even if I stepped on or even floored the gas pedal. No change at all in response to the gas pedal. It sat there for 15-30 seconds, barely running. I tried slipping into Neutral - no change. When the light turned green, it almost didn't move the car forwards, then it sort of crawled along. Then, mid turn, it came back and ran like nothing had happened. I took it on another drive afterward - it ran perfectly.

My daughter reported it doing the same to her, so it's not a one-time issue, it's intermittent. Also, sometimes it feels like it cuts out for a second sometimes, then comes right back.

Car has 329k miles on it. Ignition is all pretty new - about 10k miles ago - Bosch cap and rotor, Volvo plugs, Bougicord wires. Coil was replaced with new Bosch ~50k ago.

The only code was 4-4-3 on A2. This is for TWC efficiency - I need a new CAT. It was nacent, so not enough to light the CEL. It's been setting that code for a long time, this is a new issue, so I think unrelated. No other codes.

I haven't been able to measure the fuel pressure when it's stumbling. (Working on that.) Gas tank was full - just filled up. I disassembled and inspected the fuel pump relay - it looked fine, contacts looked clean and new. Plus, I would expect a fuel pump relay failure to be binary - it either runs or it doesn't.

Other service of note: my COVID project was to replace the turbo CRHA on this car. It's done ~1300 miles since that was completed. (Not a lot, I don't have cause to drive much these days!)

An additional issue: sometimes when it idles for an extended period, when I drive off it will emit a large cloud of smoke. Not every time, but sometimes, maybe it's done it 3-4 times. This started with the turbo replacement; I suspect something is leaking into somewhere, pooling, and burns off when I come off idle. But I haven't figured out where yet.

I am wondering if this stumbling is symptomatic of the cam sensor or crank sensor failing, although I usually think of them as binary, as well, either they work or they don't.


So, what would cause a really bad stumble for ~30 seconds, then come back and run strong with no apparent lingering effects. Any ideas? Thanks for the input!

Roger

[erikv11]

Here are some comments though they may not address all elements of your nicely thorough post:

- My first thought is a torn/cracked intercooler hose that doesn't always leak but when it does, way too much unmetered air gets in. Especially if the problem is most common at idle.
- Fuel pumps often die an intermittent death. How old is the one on your car?
- I once solved an intermittent stumble on one of my 850s by replacing the throttle position sensor (TPS). In this case the stumble would be consistent with a specific location in the gas pedal e.g. at idle, at a light touch on the pedal, etc. Seems less likely here.
- Cam and crank sensors should be binary, agreed.
- The smoke you describe sounds like classic leaking valve stem seals, especially given the 330k mileage on the engine (presumably the coincidence with turbo repair is just that, coincidence). Is it blue in color?

[Roger_850T]

Hi erikv11, thanks for the input!

- My first thought is a torn/cracked intercooler hose that doesn't always leak but when it does, way too much unmetered air gets in. Especially if the problem is most common at idle.

I'll check, but I don't think this is likely. When I took it for a test drive afterwards, I hit the boost pretty hard, and it had no issues at all, ran perfect. If it was going to leak, it would have definitely leaked then. I've had a hose pop off before, but stay in place - for me that has usually had "poor performance on acceleration" symptoms.

- Fuel pumps often die an intermittent death. How old is the one on your car?

That's a good possibility. Last replaced at 152k miles, in 2005. That means this one has done 170k miles, so more than the first one did. Would a dying fuel pump deliver minimal fuel for a little bit, then come back to full flow?

- I once solved an intermittent stumble on one of my 850s by replacing the throttle position sensor (TPS). In this case the stumble would be consistent with a specific location in the gas pedal e.g. at idle, at a light touch on the pedal, etc. Seems less likely here.

I replaced the TPS in 2016, ~50k miles ago. I had similar symptoms. This is different, it's like something just switches off then swithces back on again.

- The smoke you describe sounds like classic leaking valve stem seals, especially given the 330k mileage on the engine (presumably the coincidence with turbo repair is just that, coincidence). Is it blue in color?

Tough to tell. I think more white, but I only see it in my rear view mirror as I drive off, so not much time to study. I'll look closer the next time it happens. Would it only do it after an extended idle?

Roger

[850oldschool]

I'd have somebody else drive it and try to get a whiff of the smoke. Oil, fuel, and (god forbid) anti-freeze all have very distinctly different smells.

[erikv11]

I'll also throw in the generic "bad vacuum line" suggestion to the unmetered air idea because it is vague and easy to speculate about. You just have to poke around to prove/disprove it. something to look at anyway.

Leaking valve stem seals can give smoke in three (related) settings:
- after extended idling
- first thing in the morning after the car sits all night and oil drains down past them (this is super common)
- when you hit the throttle after a long downhill coasting

Fuel pump is always go or no, it fits the symptoms well. I'd expect frequency to increase if the fuel pump is going out. Tf this has been going on for months then ehhhh not sure fuel pump is a good idea (unless the car sees little usage).

[Roger_850T]

I measured the fuel pressure at a nice, solid 2.5 bar, 37 psi. It fluctuated nicely when I revved the engine, and the regulator responded to the vacuum fluctuation. I subsequently checked the diagnostics, and realized that the spec is actually for 3.0 bar, 43.5 psi. Hmmm...

Roger

[misha]

Yes...BUT.
'95 turbo have vacuum controlled FPR on fuel rail.
That said....
It should have 43.5 psi at 1st prime without starting the engine
-around 35 at idle
-and again 43.5 psi at full throttle

It doesn't have constant 43.5 all the time like M4.4 cars.

[BlackBart]

I would second the idea of vac line leaks or intercooler piping leaks. I once had a cracked intercooler case on the '94 850T. It stumbled and sputtered and ran incredibly rich, blew black smoke. It would idle, but not well, and any throttle would choke it and revs wouldn't climb.

[Roger_850T]

Yes...BUT.
'95 turbo have vacuum controlled FPR on fuel rail.
That said....
It should have 43.5 psi at 1st prime without starting the engine
-around 35 at idle
-and again 43.5 psi at full throttle

It doesn't have constant 43.5 all the time like M4.4 cars.

Good point, thanks for keeping me straight! I'll double check before jumping in and replacing things, that might be what I was seeing. (And plausible, since the car runs great except for these periodic episodes...)

Roger

[abscate]

Yes...BUT.
'95 turbo have vacuum controlled FPR on fuel rail.
That said....
It should have 43.5 psi at 1st prime without starting the engine
-around 35 at idle
-and again 43.5 psi at full throttle

It doesn't have constant 43.5 all the time like M4.4 cars.

Good point, thanks for keeping me straight! I'll double check before jumping in and replacing things, that might be what I was seeing. (And plausible, since the car runs great except for these periodic episodes...)

Roger

Roger..one of the most bizarre episodes of vacuum leak troubleshooting happened to me on my T5

Cause...completely rotted purge vacuum hose under the intake manifold, which splits to a T ( one side to snorkel for vacuum under boost, the other side to purge valve, the rotted part provides purge vacuum under normal running)

Symptom..severe chugging, running on 3 cylinders during first 60 seconds cold running. Suddenly clears amd then car ran well.

Documented here

viewtopic.php?f=1&t=84552&start=20

There is no substitute fir removing and replacing every vacuum hose on high mile/age cars

+1000 points for innovative use of the slash!