Advice/Opinions Appreciated: Extremely Rough Idle on 3.2 L - Time for Top End Work?

jhoagg · 12-31-2014, 12:51 PM

I have read the posts on rough idle problems on 3.2 L engines (and the fixes), but wonder if this is different. Engine has around 100K. Starts right up, and proceeds to idle extremely extremely roughly (both warm and cold). Warm idle is right around 800. When run up above idle it seems OK, and seems to pull strong up the rpm range when the car is driven. No stumbling or other bad stuff.

Would you suspect a top end problem like busted head stud, burnt valve or other even though it runs much better at higher rpms? Is a really terrible idle symptomatic of top end problems? I am thinking a leak down test is indicated, and a look for a loose head stud.

Here's the little I know:

1) It's firing on all 6 cyls. The plugs have been in quite a while, and all look like very normal wear (no dry or wet fouling, etc) - except the center insulator on one plug which is still quite white, instead of the tan/grey color of all the others. But the rest of that plug looks like all the others with normal wear and color.

2) Pulling off the oil cap decreases rpm as expected and makes the idle even worse.

3) No question it has been a long time since this engine had a new fuel filter, valve adjustment, retorque of manifold studs, etc, etc.

Stonestreet · 12-31-2014, 01:05 PM

A very skilled Porsche mechanic told me that any time you have an engine (with Motronic) issue, to go through the DME test plan and 90% of the time you'll find the cause. I don't have a copy of the test plan, though I'm passively looking for one. Either way, I found it very reassuring advice. Most of my issues have been a bit of corrosion on a contact or a loose connection somewhere.

It could be anything but I imagine that a need for valve adjust would show up not just at idle, same with fuel filter, no?

Anyway, if you can get your hands on the DME test plan, that's where I would start.

Justin@Athens · Justin@Athens's Garage

Quote:

Originally Posted by jhoagg

Would you suspect a top end problem like busted head stud, burnt valve or other even though it runs much better at higher rpms? Is a really terrible idle symptomatic of top end problems? I am thinking a leak down test is indicated, and a look for a loose head stud. .

From my understanding yes. Broken stud might very well be your problem. Pull the valve covers, inspect for broken stud. Adjust valves at the same time.

jhoagg · 01-01-2015, 08:26 AM

Thanks. I'm going to look for a busted head stud next.

Has anyone else experienced that same thing - really bad idle with OK performance above idle with a broken head stud?

BTW - I think I have found a copy of the DME test plan for Carreras. I'll post if that might be helpful for other owners.

Stonestreet · 01-01-2015, 05:18 PM

Quote:

Originally Posted by jhoagg

Thanks. I'm going to look for a busted head stud next.

Has anyone else experienced that same thing - really bad idle with OK performance above idle with a broken head stud?

BTW - I think I have found a copy of the DME test plan for Carreras. I'll post if that might be helpful for other owners.

It's my understanding that our host would rather we didn't share pirated material on their forum, but if you've found a place to access or download a PDF of it (legally of course) I'd be more than interested. Also I would highly recommend doing the test plan before pulling things apart. That said, you are well overdue for a valve adjust.

Arne2 · Arne2's Garage

Quote:

Originally Posted by jhoagg

BTW - I think I have found a copy of the DME test plan for Carreras. I'll post if that might be helpful for other owners.

Bearing in mind the above caveat, if it is legal for you to share what you found, please do. I don't have need of it right now, but who knows what the future may bring?

DRACO A5OG · DRACO A5OG's Garage

Hmm???

Two things stand out in your comments:

1. 1 discolored plug, tells me it may be a lean situation on that plug, maybe clogged or weak fuel injector.

2. Fuel Filter not changed, well this could be the problem. Many had found this to be the culprit.

If you want to be certain, test the fuel pressure. purchase a cheapo tester at HB and verify.

+1, While you are in there, doing your valve adjustment check the studs. Are you burning more than normal levels of oil? Why not do a wet/dry compression test or even better a leak down test?

Quicksilver · 01-02-2015, 08:24 AM

The DME Test Plan is probably the least recommended suggestion and it should be the first thing suggested for these cars. (Replace, replace, replace = fail. Test, test, test = succeed.)

Lets look at the physical conditions where the issue is happening:
- Low RPM
- High vacuum
- Low fuel flow

The fuel is fed by an electric pump so it has a fixed supply capability. If will run at high RPM under load then there isn't a fuel supply issue at idle. (Over pressure could be an issue...)
A spark jumps easiest in a vacuum and as pressure rises creating a spark becomes much more difficult. The higher the RPM the less time the ignition system has to generate the spark energy. If it runs under throttle at higher RPMs then idle wouldn't be a problem.

The biggest possible issues for rough idle would relate to the fact that idle is under a high vacuum condition. Look for vacuum leaks: Hoses, intake manifold gaskets, etc...

Other lower percentage issues? Broken head stud could cause an issue but you should be able to hear that kind of leakage. Dripping injectors could cause more idle issues as opposed to high RPM problems.

mreid · 01-02-2015, 08:39 AM

Your car is crying for a thorough inspection, valve adjustment, and tune up. Do that, replacing all if the normal maintenance items (filters, cap, rotor, plugs, wires, etc.) and report back.

r-mm · 01-02-2015, 03:47 PM

Agree you need to do the very basics as outlined above, but do them with eyes wide open, super bright work lights everywhere and don't take anything for granted. its amazing what you see when you're not just in get it done mode but really look at the car with fresh eyes.

When you say it runs well "when run up above idle" are you saying it revs like a dream from say 1000-3000? If you tap at the gas over idle it feels right as rain?

jhoagg · 01-04-2015, 08:11 AM

Thanks all for the advice and recommendations. I will be able to get seriously started on this next week, and I know there is basic maintenance to do first, as well as quickly look for broken stud.

r-mm: yes, the engine seems to run fine (both under load and not under load) once rpms are above about 1K. That is what seems weird to me.

Quicksilver: Great points, and it would be nice if my basic problem is vacuum leaks, even though maybe a real pain to find. Doesn't it make sense to test first with a vacuum gauge - to see if there enough leakage to cause such lousy idle? If so, where could I connect it? I have seen reference to a test port on the manifold, but can't find any pictures or other to show this. Bentley and other sources don't seem to address this on the 3.2.

john walker's workshop · 01-04-2015, 10:14 AM

Carreras often suck in one of the intake gaskets and cause that cylinder to drop at idle and low rpm, due to too much air in the mix. Pull off one plug wire at a time and see if the idle drops off or not. A compression and leakdown test would be good. Look under the distributor cap for carbon tracking.

Quicksilver · 01-04-2015, 01:46 PM

Quote:

Originally Posted by john walker's workshop

Carreras often suck in one of the intake gaskets and cause that cylinder to drop at idle and low rpm, due to too much air in the mix. Pull off one plug wire at a time and see if the idle drops off or not. A compression and leakdown test would be good. Look under the distributor cap for carbon tracking.

To add to this:
When mine became a "great sucking vacuum leak™" I figured out which three cylinders (yup 3 of them!!!) had a problem by putting the car up on jack stands and starting it cold. I then took an IR thermometer and measured the temp of each of the exhaust ports as it started to warm. Three of them were quickly more then 100° warmer then the others. Checked the cold ones for other easy to check issues like injector flow and spark then I chased after the intake gaskets.

----
If it is the intake gaskets:
You will need 12 gaskets because each port has 2 gaskets. One above an insulating plastic spacer and one below.
ALSO! With the age of the car the plastic spacers will almost certainly be crumbling when you try to clean them (just like mine were). So before diving too far into the job make sure you order the 12 gaskets and 6 plastic spacers.

If this is your issue, it is a major "while you are in there" opportunity. Check the fuel hoses very carefully and go through all the rubber hoses looking for cracks and vacuum leaks. I had Mese Hose here in the OC replace the rubber fuel lines with a more permanent line. I also found a couple cracked vacuum lines and the main breather hose cracked when I tried to reconnect it. Turns out the main breather line isn't supposed to be a hard, almost plastic like hose! A new one is VERY flexible rubber.

I also did a major cleaning while I was in there. I used the gaskets as a stencil to cut some masonite port block-offs so I could go to town and not worry about junk going down the intakes.