940 Rough idle, fuel pressure issue?

I've been trying to diagnose a rough idle on my '91 940 turbo for a while now, and I can't figure it out. I've checked very carefully for vacuum leaks, and I can't find anything after smoke testing as well as spraying around starting fluid. I've replaced all the vacuum lines, crank sensor, coolant temp sensor, fuel pressure regulator, fuel filter, air filter, cleaned the MAF sensor, and other things that I can't remember right now.
I want to check the fuel pressure, but my rail doesn't have a schrader valve, so I'd have to get creative. I'm planning on swapping over the rail and intake manifold from my 240 eventually anyway since that has a valve, plus I can delete the cold start injector to clean up some clutter.

Anyway, it's only a problem at idle. It runs a little rough, the tach moves very little, but my boost gauge (Bosch gauge, the factory one is disconnected) bounces violently. It stays right around 20 in-hg at idle, but the needle vibrates. It also bounces in the same way at max boost (around 11-12 PSI currently set on my MBC, and no that's not the problem, it does the same thing without that) but the boost pressure doesn't bleed off like there's a leak.

One thing I noticed while messing around with it at idle, when I pulled the vacuum line from the FPR, nothing changed. I heard and felt the vacuum from the hose, but the idle didn't change, and neither did the bouncy boost gauge. I found that really strange.
I had replaced the regulator with an aftermarket one, but honestly the car felt a little more sluggish with it, so I just reinstalled the original Bosch FPR. I'm not sure that the aftermarket one was the same 3.0 bar rating as the original, so maybe that was the issue... who knows, it was a cheapo.
Due to this, I'm thinking maybe my fuel pumps are weak? I'd really like to see the fuel pressure, but I'm not sure how to go about connecting a gauge. The car pulls well on the road, which is why I'm confused as to why there's such a rough idle. I know red blocks aren't the smoothest engines, but I don't think this is right.

It's worth mentioning that the engine mounts are pretty worn out, but I don't know if that would cause the tach to wander slightly and cause the bouncing boost gauge. The engine definitely shakes.

Have you done a compression test? A bouncing vacuum gauge needle typically indicates a poorly sealing or burnt valve which would also produce a rough idle.

If you still think you need to check fuel pressure I have the factory gauge and adapters available to rent.

I have not checked compression. I thought about it the other day, knowing I should, but it slipped my mind. I'll do that tomorrow.

It runs really well otherwise, but the bouncy gauge at full boost is also a little weird/concerning. I'll post up results of the compression test tomorrow.

Compression tested it yesterday... cylinder 1 was a little low.
1- 125 PSI
2- 160 PSI
3- 175 PSI
4- 170 PSI

I'm going to do a leak down today on cylinder 1 at the very least and try to see what's going on. 125 is basically the minimum I'm seeing is acceptable for these engines, but it's WAY lower than the other 3 cylinders.

That's cause for concern. The difference between the highest and lowest compression cylinder should not exceed 28 psi. The 125 and 175 readings by themselves are off the scale.
You could try playing the engine flush Russian roulette with your block. But this would probably be a better time to be relieved that there's less than 50 bolts between you and that head sitting on your desk, and you only have to take off the 13mm socket a couple of times.

I have a feeling there's a very minor head gasket leak between cylinders 1 & 2, or possibly an issue with the valve(s) on cylinder 1. I'm going to do a leak down once this passing shower stops and see where it's leaking. I might do an intake Seafoam treatment.

I wouldn't be opposed to pulling the head either, it's easy enough. The car only has 174k miles, I'm surprised it has low compression on a cylinder. Doesn't use any coolant, but it has taken down about a quart of oil over the last 1k miles. But it does have an oil leak from the dizzy shaft seal.

Did the leakdown test on cylinder 1 and 2, and it didn't seem to tell me much. My tester is a piece of crap old HF Pittsburgh one, so it's not the greatest at giving good readings either. It showed it in the red section, and I heard air initially through the intake. However when I tested again, I could only really hear/feel air from the oil dipstick and cap. It wasn't a ton, but it was definitely there.
Nothing from the coolant expansion tank or cylinder 2. Seems to be worn rings if anything, maybe I'll do a wet compression test tomorrow and see if compression goes up at all.
Cylinder 2 showed a higher leak down as well, but again with the cheap gauge who's to know how accurate that is. I didn't feel air anywhere with that cylinder, just a very small amount from the oil cap (normal blow by).

I did a Seafoam treatment through a vacuum hose, sucked down half a can and let it sit for 10 minutes, then took it on a hard drive on the highway in 3rd gear for the next 10 minutes. Definitely puffed out some smoke, but it runs exactly the same. I was hoping there was carbon buildup on the valves or something preventing them from sealing, but this doesn't seem to be the case.

Fuel in the vacuum hose to the fuel pressure regulator?

lummert said:

Fuel in the vacuum hose to the fuel pressure regulator?

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There's a fuel smell, but no fuel. I replaced the original FPR after it felt like the car was slightly down on power with the replacement I installed.

The strange thing though is that there is no difference in idle when I pull that vacuum line.

bmdubya1198 said:

There's a fuel smell, but no fuel. I replaced the original FPR after it felt like the car was slightly down on power with the replacement I installed.

The strange thing though is that there is no difference in idle when I pull that vacuum line.

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You getting vacuum from that line?

lummert said:

You getting vacuum from that line?

Click to expand...

Yes, there?s definitely vacuum coming through it.

Compression tested again after chasing the threads of the #1 plug (got a little messed up, but I saved it!) and now see 120 PSI. The engine wasn?t up to temp though, so that probably explains it.
However, I then tested with a small amount of oil in the cylinder and it went up to 135 PSI. Maybe this could point to a ring issue?

Checked it again with about 1/4 ounce of oil at operating temperature. Dry reading was 120, then the wet reading was 140.

It occurred to me last week that my dizzy seal was leaking. So, after cleaning up the dizzy and replacing the shaft seal (the Baum tools distributor jig is a must-have, by the way. Turns a massive pain in the a** into easy work) it runs no differently. I also replaced my injector o rings, and discovered that two were pretty mangled. Also have a couple cracked pintle caps that I had to put back since I didn’t have any replacements, is that going to cause problems?

Other than that, the only thing I haven’t touched is the O2 sensor, but I really don’t think that’s a problem. I’m not opposed to pulling the head, but I’m afraid that everything on that end is fine and there’s actually an issue with the rings. When I do a leak down, I initially see smoke from the turbo outlet (car was just running, then I removed the intercooler piping for easier access to the cylinder 1 plug) and a little from my intake (a cone filter). But after that, I really only feel and hear leakage from the oil cap. Maybe the valves aren’t seating well?

I'm considering pulling the head in the next couple weeks now that I'm planning on dropping in an iPd turbo cam that I just ordered as well as a 16t. I have a freshly redone 530 head off my 240, and I figured I might as well put it to use since I have all the parts anyway.

So my point... what are the odds that the rings on cylinder 1 are the problem rather than the valves? When I do a leak down test, I get air through the turbo of all things with cylinder 1 at TDC. My cheapo HF gauge shows pretty high on the leakage gauge. But when I turn the engine clockwise a little bit, the number goes down into the "green" zone but I still get a little bit of air through the turbo. It's also worth noting that the turbo smokes when I do this, and I have the outlet elbow disconnected for better access to the spark plug. It comes out of both the outlet and the intake (just have a cone filter on this car right now). I feel like maybe the exhaust valve on cylinder 1 isn't really sealing right.
I also hear leakage through the oil cap, but I've never had an engine not do that.

Update on this problem... I've been experiencing some decreased power now. I figured it was due to running the AC, but last night I had it off and the car hesitates on heavier throttle before finally moving. It backfired once too while accelerating. I'll check the injectors shortly.
Just out of curiosity, I pulled the dipstick, and it's puffing a fair amount of smoke. Nothing too dramatic, but it's there. I don't know if there's an issue with the rings on cylinder 1 or what. PCV is new, only been on there for 1,500 miles at the most.

With your oil-in-cylinder test I would suspect valves/gasket over rings. The 125psi is definitely a problem but probably not rings. Is there a leak down test in there somewhere?

TestPoint said:

With your oil-in-cylinder test I would suspect valves/gasket over rings. The 125psi is definitely a problem but probably not rings. Is there a leak down test in there somewhere?

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I'm hoping for valve/HG issues, but the blow by is concerning. I mean, it is a red block after all, so it may be totally normal, but I just want this thing to idle smoothly and not backfire.

I did do a leakdown test, at least two or three actually. With cylinder 1 at TDC, I get smoke from the air filter (currently just a cone filter) and the turbo outlet (I had the intercooler hose off to more easily access the spark plug) for a few seconds, then it clears up. I hear a little air through there, then the usual air through the crankcase with the oil cap off.
I don't remember the exact percentage. My gauge is a cheap HF piece of crap, and I don't trust the readings one bit. It was still in the "green" though, IIRC.

Wonderful... I just spent the past day and a half replacing the cylinder head, and it runs the same... if not even worse. Boost gauge still twitches around 12-15 in-hg and it idles horribly. It drives okay otherwise, but still has occasional hiccups. I can't find any vacuum leaks, which seems ridiculous... you'd think I would have a pretty big one. This just makes no sense, and I'm beyond frustrated with this car.

Glad I wasted $200 on that cam...

Anyway, does anyone have any idea what's wrong? Maybe a torn CBV?

Also, the valves and pistons all looked perfectly fine. Still have a sh** load of smoke and pressure from the crankcase when I take the oil cap off. I'll post some pics later.

Here are some pics of the cylinder wall of cylinder 1, then 4 for comparison. Looks decent, the cross hatching is still visible, albeit less wso than the other cylinders. https://photos.app.goo.gl/NQeUvV8ZTzyWpJVz5
No visible issues with the valves.

I swapped a fuel rail from my 240 that has a schrader valve on this car so I could check fuel pressure. It primes to 20 PSI, but it bleeds off pretty quickly. Within 30-45 seconds, it's back down close to 0. When running, it hovers right around 37 PSI. This is with the FPR vacuum line connected, I didn't check it with the hose disconnected. I will check that tomorrow.
Looks like that's a problem though, no?