engine vibration - need help at what to look at

[redcoupe86]

I have a vibration at idle (most pronounced at and around 800 rmp) and you can smell a rich exhaust. You can see and feel the engine rocking back and forth, and can feel it in the seat of the car.

Things checked and replaced up to this point.

New O2.

New Plugs

New Ignition Wires.

Compression test (cold): all withing 10%.

Old plugs all the same. No oil or black plugs, all are light grey/brown and you can see that about 70% +/- of white part of the plug as white - no deposites.

Jacked up engine, mounts didn't move. Car went up as soon as jack hit engine.

Check Rotar and Cap - seem good.

Any suggestions at what to look for?

Will be taking off valve covers in a week or so checking for broke head studs. Anything else I should check while I'm there?

[Jim Sims]

Check for correct placement and complete seating of ignition wires on distributor cap. Check to ensure ignition wires (boots/connectors) are correctly seated on sparkplugs.

[redcoupe86]

Originally posted by Jim Sims
Check for correct placement and complete seating of ignition wires on distributor cap. Check to ensure ignition wires (boots/connectors) are correctly seated on sparkplugs.

Thanks for the advice. I have done this, and the problem persists.

Would idle mixture come into play here? I read that you need a CO analyzer to set correctly - im not sure what a CO analyzer costs or where to get my hands on one. Im of the understanding that the mixture screw only controls the mix at idle. Does it also controle the air/fuel at any other position in the throttle?

[redcoupe86]

Bump

[Geo9146GT]

Does the roughness/shaking persist off idle?
does it run smoothly with good power at speed?
have you done a leakdown test?
If so, what are the readings?
did you test the O2 sensor and other parts first, or just replace them on spec?

[kjb]

How about checking the idle mixture?

/ J

[ianc]

Are all the cylinders firing at idle? If you disconnect the ISV, then start pulling injector wires off one by one, do you get one cylinder that doesn't affect things much?

ianc

[redcoupe86]

Originally posted by kjb
How about checking the idle mixture?

/ J

I'd like to, but im not sure what the best way is.

[redcoupe86]

Originally posted by Geo9146GT
Does the roughness/shaking persist off idle?
yes

does it run smoothly with good power at speed?
i mean, you cant really feel it, but if youstand behind the engine and rev it you can see the vibration

have you done a leakdown test?
no, just the compression test.

If so, what are the readings?
N/A

did you test the O2 sensor and other parts first, or just replace them on spec?

just replaced them on spec. The engine had 117000 miles and the paperwork didn't show they had been replaced.

[Geo9146GT]

you may have a misfiring cylinder at idle, that's why i asked if the engine runs smoothly at greater throttle openings. This could be due to a vacuum leak, which would only affect the engine at idle.
a quick way to tell is to spray alcohol based brake cleaner over the intake system, keeping away from the air filter intake horn, and see if the idle jumps up. the suggestion to pull off the injector connectors is a good, if time consuming, one. If you don't have a vacuum leak and you find that one cylinder does not respond to having the injector connector removed, then I
would pull that injector out, reconnect it and have someone crank the engine to see if it sprays fuel. If it does, I suspect you have a real PROBLEM. I would recommend a leak-down test and/or ign scope test to determine what's up with that cylinder

[redcoupe86]

Originally posted by Geo9146GT
you may have a misfiring cylinder at idle, that's why i asked if the engine runs smoothly at greater throttle openings. This could be due to a vacuum leak, which would only affect the engine at idle.
a quick way to tell is to spray alcohol based brake cleaner over the intake system, keeping away from the air filter intake horn, and see if the idle jumps up. the suggestion to pull off the injector connectors is a good, if time consuming, one. If you don't have a vacuum leak and you find that one cylinder does not respond to having the injector connector removed, then I
would pull that injector out, reconnect it and have someone crank the engine to see if it sprays fuel. If it does, I suspect you have a real PROBLEM. I would recommend a leak-down test and/or ign scope test to determine what's up with that cylinder

I guess I need to look into how much a leak down test costs and where to get one. There are no Porsche shops in my area that I know of. There is one place that works on european cars in general I can look into.

I don't think I have injectors that are bad or a cylinder not firing b/c all the spark plugs looked the same. I would have thought that if a cylinder wasn't firing it would be black and wet.

Reguardless, I DO need a leak down test!

[Geo9146GT]

If you have an injector not working, you get no fuel in the cylinder.
You would not have anything to get the plug wet with. I think you should definitely do a cylinder kill test - either by pulling the spark
plug connector off each plug (not without a proper tool) or disconnecting the injectors successively. I also think the vacuum leak test should come first, it is the quickest way to eliminate the possibility that this is just a leak in the intake system .

[javadog]

I think a leakdown test is the least of your problems and most likely a waste of perfectly good money. Try the simple stuff first. You said the old plugs looked normal and all the same. That would rule out a dead cylinder. You say that exhaust smells rich. With a cat bypass, you will have a strong smell to the exhaust. Are you sure it's rich? I have a few cars that run without a cat and some of them will make your eyes water.

Make sure you have no vacuum leaks. Find a shop with an evaporative emissions tester and have them check for leaks. Intake leaks are notorious for causing a rough idle. The leak may be nowhere near the engine. Remember, you have evaporative emissions lines at the front of the car.

If you have no leaks, check the CO and see if you are within specs. Then check the ignition timing at idle.

If at this point you still have a rough idle, there are a few more things to check.

If you check for broken heads studs you might also check your valve guide wear at the same time.

Good luck,
JR

[redcoupe86]

Originally posted by Geo9146GT
If you have an injector not working, you get no fuel in the cylinder.
You would not have anything to get the plug wet with. I think you should definitely do a cylinder kill test - either by pulling the spark
plug connector off each plug (not without a proper tool) or disconnecting the injectors successively. I also think the vacuum leak test should come first, it is the quickest way to eliminate the possibility that this is just a leak in the intake system .

Good advice - I'll do it. One question though, If Im getting light grey/brown deposits on the plug, wouldn't that indicate that the I'm getting combustion?

Just for info my compression test (cold remember - this isn't the most accurate as warm) are

160 167
152 150
160 160

understand that the way I had the car jack for a while made the car lean toward the 167 number, though I have no idea if that would have affected it one way or the other.

[ianc]

One question though, If Im getting light grey/brown deposits on the plug, wouldn't that indicate that the I'm getting combustion?

At some point during the plug's life, yes. Did this condition just occur recently, or has it been there for awhile?

If the engine runs rough at idle, but smooths out at higher RPM's (sometimes it can be difficult to tell this), I'd be more inclined to suspect a vacuum leak, as the false air would be much less relevant vs. the total air volume consumed by the engine. If it's rough throughout, I'd think more that one cylinder is having fuel\ignit problems.

ianc

[bkreigsr]

I once bought a set of Bosch plugs where one plug had a broken ceramic insulator above the threads that I could not see. I could gap it, install it, and the insulator would slide down to contact the electrode, making the cylinder go dead. took me three installs to figure that out. I still have the ba*tard after 7 years.
just a thought
Bill K

[redcoupe86]

All good info.

Maybe i should add this. I changed the plugs and the car still vibrates. The old plugs where the grey/brown plugs, but the new plugs didn't change anything.

However, as I mentioned before, it does vibrate when I pull the throttle - just really fast and you can BARELY SEE IT, and you don't really feel it when you're driving, you only feel it say below 1500 RPM or right when you crack the throttle. Now this COULD be normal, but since I dont have any experience with other engines like mine, with my exhaust, its hard to tell.

Sometimes I think my exhaust note exagerates my perception of what Im seeing and feeling. The exhaust has a definate "pulse" if you know what I mean.

My scare is this, that under compression on acceleration the compression numbers aren't so close and one of the cylinders is much further off causing a vibration. This is why I need to to the leak down test. But first I'll do as suggested and do the vacuum leak search.

[bkreigsr]

if you are new to this game, maybe when you were exploring the upper rev limits you dislodged some carbon build-up from the intake runners or tops of the exhaust valves and it got caught on the valve seat. this will cause similiar symptoms as a broken stud. some over-the-counter valve cleaner-upper usually does dissolve/dislodge it while driving
maybe
Bill K

[redcoupe86]

I got to thinking last night, could it be possible in a 3.2 for the cam chains to stretch causing timing t be a little off. If so is there an easy check or do the cam covers have to come off?

[javadog]

Cam chains can stretch but that has nothing to do with your rough idle. Checking and adjusting the cam timing is a fair bit of work and, yes, the covers do need to come off if you want to adjust it. You can check it without removing the covers.

JR