bmdubya1198
Active member
- Joined
- Aug 24, 2016
- Location
- Charlotte, NC
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.
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.