Hi guys,

I'd like your sage advice on the following situation:

1997 850, normally aspirated, manual transmission
230,000 miles, single owner (that's me , and properly maintained the whole time.

The symptom that I'm having is that I'm getting a bit of vibration at idle only; above 1,000 RPM, everything is fine. The engine revs properly (i.e. no hesitation when applying the accelerator from idle in neutral or under load), and gas mileage is normal. Engine temperature doesn't seem to have any effect -- hot or cold, the vibration is present.

I pulled the plugs which have about 10,000 miles on them, and unlike when I changed them last, they were now carbon fouled, so I'm running rich at least part of the time, and I'm thinking at low RPM. I cleaned them up, and put them back; the symptom persists.

* I recently did the PCV system and engine mounts (this was the second time, with free replacement parts from the excellent folks at FCPEuro :-).
* New thermostat (note: the temperature gauge is right in the middle of the range as it's always been)
* Thorough cleaning of the throttle body, and throttle plate;
* Thorough cleaning of the MAF sensor with some fancy MAF cleaner;
* I did a smoke test on the vacuum system, and found two small leaks (one of which was from the o-ring on the vacuum tree, which resulted in a snapped bolt in the intake manifold, subsequently drilled and tapped without metal swarf into the engine ;-); both fixed.
* Checked all vacuum elbows for cracking, etc.
* Did the relearn procedure on the ECU computer.
* No CEL or codes in the computer.

... but the vibration at idle symptom persists.

So now I've got an ELM327 reader coming, and intend to go sniffing around the fuel trims.

I'm thinking that it's likely the TPS sensor or the MAF sensor, or maybe *shudder* a cylinder starting to leak.

But before firing the parts canon at my old girl, I'd like your opinion on how best to proceed.

What are your thoughts?

Thanks in advance!

 

cap/rotor/wires? wires have a date code on them - easy enough to do a baseline tune for the ignition. I've had the experience to find corrosion on a wire under the boot to the cap so don't assume all is good. Did you clean the idle air valve as well? How are the motor mounts? is the upper torque mount still in good shape (the one over the head)?

 

Good suggestions, and I forgot to mention that 10,000 miles ago, when I replaced the plugs, I also replaced cap, rotor, and wires.

Yes, when I cleaned the throttle body and throttle plate, I also cleaned the IAC valve.

When I did the motor mounts, I also did both the upper and lower torque mounts...

 

Could very well be simple as exhaust shaking around the engine or cabin. And when you give it throttle it stops.. check hangers and for leaks.

 

The hangars are all good (i.e. replaced a couple of years ago), and on your suggestion, I just checked them.

One other thing... When I switch the engine off, just before it stops turning over it moves more than it should. Like a shudder just before all movement stops.

 

shudder sounds like some run-on (signs of running rich). So if you've confirmed your spark is good, next is fuel delivery. you can test pressure at the rail via the shraeder valve to check for too much pressure. The fuel pressure regulator (damper?) hides under the intake manifold and check for a problem with the fuel return line. Other things that may impact fuel trim besides the MAF include the front O2 sensor - which can get thrown off by an exhaust leak - so check around the flex pipe and the EGR tube. I would think that other sensors like the MAP would throw a code if wonky but you never know. When you checked the plugs, were they all equally bad? if yes, that would rule out a leaky injector, if one was bad then I'd consider a swap out ...I can sell you a used set of orange injectors :-)

 

Weak cylinder? I had a 300,000 miles car that developed a low speed shake like that. I was not interested in fixing it, though.

 

You said MPG was good so that would mean fuel trims are even?? I really have no idea

 

I don't (yet) own a suitable fuel pressure gauge, and the local places that loan them out don't have the schrader plumbing to hook it up, so I did a few other things in the hope that I'd get lucky...

I pulled the plugs again, and they're lightly carbon fouled after the cleaning a week ago, and a week's worth of local driving. With the plugs out, I did a cold dry compression test and got the following results:

Cylinder 1: 200 PSI
Cylinder 2: 205 PSI
Cylinder 3: 195 PSI
​​​​​​​Cylinder 4: 205 PSI
​​​​​​​Cylinder 5: 200 PSI

I then pulled the fuel rail/injectors (very easy to do when you have viton o-rings and they've been liberally coated in silicon grease ;-)), and cycled the fuel pump 8 times or so, and found no leaks from any of the injectors.

So, based on the above two tests and the fact that the plugs all seem equally fouled, I'm going forward with the assumption that this is a fuel/air issue, and not a cylinder on the way out.

FYI, this car has the pressure regulator located just in front of the fuel filter (roughly under the rear passenger seat), and has a fuel damper on the underside of the fuel rail.

I then pulled the vacuum elbow off the fuel regulator and started the car. There was no change in the idle behavior, and I didn't hear or feel any "sucking" on the vacuum elbow. I'm not sure how much vacuum I should get at idle, but this didn't seem right to me. I have a mityvac hand vacuum pump which has an integrated manometer, and it read zero. Might this be because of the EGR valve which is between this vacuum line via the canister), and the intake manifold. This I find confusing because it would mean that when the EGR valve is not open, that the FPR will not get any vacuum...

Then, thinking that there must be a vacuum leak somewhere to explain the no vacuum at the fuel regulator elbow, I re-smoked the vacuum system and found no leaks whatsoever, and I paid particular attention to the canister -- nothing.

So what should my next move be?

Thanks!

 

I realize this is an old post but did you ever find the problem/fix?

 

I don't know if you've fixed your issue yet, but I had the same issue the past month, in addition to hard warm starts. After checking all my other sensors and fixing all the vacuum leaks, I pulled the trigger on buying a new MAF since I don't have any junk yards near me, and it fixed it. Ran fine at speed but struggled to get the right fuel mix at idle before.