Bill Ball

UPDATE: Read through if you want, but in this case it was the MAF. MAF failure only affecting idle (and slightly off idle) was unknown to me prior to this event. Then only to find with a borrowed MAF, a HIGH idle (950+), all cured by a JDS Porsche rebuilt unit.

You'd think I would know the likely cause right off, but I'm not so sure. While turning to get on a freeway on-ramp last night, my trusty 89 stumbled, almost died, stumbled a bit more, feeling like it had lost power, then finally responded to the throttle and seemed almost normal accelerating and cruising at freeway speeds. When I got off at my exit and stopped at the stop sign, the idle was very low and rough. Playing with the throttle I could keep it from dying. It would stumble a bit on throttle but then accelerate and cruise well or close to it. At home I disconnected the battery ground briefly which sometimes helps by resetting the LH if for some reason some of the stored adjustments get corrupted. No change. I had recently changed the O2, so I disconnected it. No change.

This morning the car started and idled OK for a few seconds then began to idle low and rough. It seemed to rev freely and OK on throttle. Did the same thing each time I started it. I ran Theo's Porsche Diagnostic and it showed no faults. I checked coil leads, as I had a corroded one (replaced now) a while back and it has been raining. They are fine, and I would expect a misfire rather than bad idle anyway.

It doesn't strike me as a MAF failure because it revs well enough. Same for the new fuel pump installed last week.

I suppose if the idle stabilizer got stuck closed , that would account for the idle, but the rather marked stumble off idle last night doesn't quite fit. Today, I can't say it is stumbling off-idle. I should try some WD.

I have a spare LH I need to retrieve. I suppose it could be some odd form of LH failure. My current LH is a 10 year old Porsche rebuilt (pre Speake).

Any other trees I should bark up?

Leon Speed

Sounds like overfueling at low rpms, or maybe a missing cylinder. Might as well be the MAF of LH. But before going that route, in addition to the items you pointed out, maybe check the Temp II sensor? Also check the throttle plate switch for idle signal. Did you just read the faults with the tool or did you go through all "checks" (idle signal etc). Also cycle the fuel injectors with the tool and listed at each one with a stethoscope. Long shot: a fault in the ignition protection system.

IcemanG17

Idle stabilizer valve....its probably dirty causing it to stick at times....you can try the easy wd40 trick to see if it frees it up a bit..... This would improve sharky's low idle for a short period of time (think days)...but a fresh valve made it rock solid stable...no more low rpm hunting that I hated

Bill Ball

This is far more rough than 1 or 2 missing cylinders. These cars run remarkably well on 6 or 7. Even 4 will run and idle, although a bit lumpy. The low RPM is dramatic.

No black smoke or other signs of richness. The idle surges and a few times went over 1000, suggesting leanness, hence my suspicion of O2, but O2 is not read on cold start for some time, and the predominant symptom is very low idle.

I checked idle switch operation with Theo's tool and Temp II read out.

Ignition protection is disabled, and it accelerates and cruises normally, which the IPS would interfere with.

Bill Ball

Yeah, that's my general feeling. I don't know how old the valve is, but I'm pretty sure it was replaced (not original). I should check that it is getting 12V and makes the "click" with Theo's test or the Hammer.

ptuomov

I got that kind of symptoms once when the MAF had popped off the rubber boot and leaked a little.

yaskota

Bill, I'm sorry your having this issue, but I'm glad you posted it. I'm having, I think, an identical issue with my '84 928S (US) L-Jet...

With new injectors and new spark plugs, idle is über rough and will die if accelerator not feathered. The roughness occurred when- ironically- at pickup and on the drive home from a shop the PO had it at (where plugs and injectors were installed)- for a variety of reasons, no way was I going to take it back!

I'd expect some tuning would be needed but this is off the charts. (Btw- at purchase one cylinder was not firing- preheating current issue; all firing fine now.) I might even post a sound clip as it is something to hear

Under acceleration- it purrs like a kitten. But at cruising speeds, it will turn rough whenever I let up on the accelerator... unless I, again, feather the accelerator. If I don't (as proven at lower speeds like the stop and go traffic before getting on the freeway?) it dies. At an innocent idle, it surges and flutters and sputters and- sounding like a high school kids muscle car- and dies sometimes with a mild death tremor.

With your post in mind, I'm going to check under the air box. I've already checked my CE panel and it's clean with a few new relays en route just in case; 14 pin and ground cleaning is "next." New vacuum lines (and ignition so color matches) are en route too. And, let's not forget, that darn green wire is also en route!

I'm not at all sure what this but I'm just a newbie and must bow to the great minds here... Incredibly curious what you find..

-tom yasko
(SF Bay Area resident living ~mostly~ in Hawaii at the moment...)

Mrmerlin

Bill here is more info see post 35 for pictures

https://rennlist.com/forums/928-foru...?highlight=ISV

From the info provided I would also want to know the fuel pressure while running and check the vacuum lines for fuel smell and swap in another MAF

danglerb

Ljet is different kettle of fish, no idle stabilizer valve, generally less smarts in the brain.

Shop "really" put in new injectors? I would check all the areas they touched, use a hose to listen to each injector for a click when running.

Suspects that come to mind are broken wires, bad connections, air leaks, misrouted vacuum lines.

*** Bill, did diagnostics show TPS working?

yaskota

I appreciate the feedback danglerb especially since I have found many of your posts to be quite informative. I'm aware of the differences, but curious as the symptoms seem pretty close to identical to Bill's. Injectors are good & yes they are new (all 8)- but once idling, I'll pull out a stethoscope for a listen as somewhere I read "new doesn't always mean working" or something like that!

The mechanic that worked on the car- a "friend" of the PO/seller- is a Porsche mechanic, but not at a Porsche shop. I'm not worried as he and I kept in touch throughout. I am aware there may be a vacuum leak (hence new hoses) but until I can get this at a steady idle- which I think might be electrical/electronic (to include electronics of fuel injection beyond the injectors themselves). One thing that did occur prior to the most recent mechanic was another house-call mechanic (before purchase) going in with a large screwdriver- similar to something I saw in the 928 factory manual- adjusting fuel/air mixture or something like that (sorry, I'm more of a driver than a wrench... but I'm trying!).

Thanks for the suspects and I will post follow-up *elsewhere* as it occurs. Until then, lacking similar examples of behavior, I'm quite curious about Bill's thread...

karl ruiter

I've had symptoms that sound similar with leaking fuel dampers. On my car I could not really see fuel in the vacuum lines, so I jumped the pump on, and put a mityvac on the vacuum port, and sure enough, there comes the fuel. Quick and easy test to do and costs nothing.

The Forgotten On

yaskota, replace the fuel dampers as they are probably original and leaking. Then check for vacuum leaks around the manifold like the gaskets at the base of spider legs.

The rear two are prone to leaking because they harden and then get loosened when some one takes off the air box mounts as those are what holds the rear gaskets down on the L-jet filter housing.

They leaked on me and I found out by checking the plugs for a whitish residue that forms when a cylinder runs lean.

For an S4 it could be any of the intake gaskets or a crack in the ISV tubing. Basically time for an intake refresh...or a refurbed maf.

John Speake

Bill, your diagnostic tool should be able to test that the ISV is responding to input signal..... on a Hammer it's in the actuator tests menu.

Check intake vacuum on the fuel damper at the front of the engine, it should be about 21"Hg at warm idle.

Bill Ball

Hi all:

John - I have your Spanner. I did not have time to test the ISV yet. I plan to check the manifold vacuum as Paul Carver suggested I may have a BIG vacuum leak.

Tuomo - the MAF is firmly seated.

danglerb and John - The TPS is working.

Stan - I don't have a spare MAF (loaned it out). And considering how well it accelerates and cruises, I hadn't considered the MAF. However, that said, I have seen some odd behavior with some 928s that resolved by unplugging the MAF and plugging it back in. Once a long time ago, George's car would only start and run with the throttle floored. Needless to say, we did not run it that way for more than a second. We tried all kinds of things and finally unplugged the MAF and plugged it back in. Problem solved. I had to retrieve my LONG screwdriver today so I can unscrew the MAF clamp and get to the plug.

Stan, Forgotten & John - I will check the dampers. Yes, they are original. However, the car starts perfectly cold or warm and no evidence of richness, so I hadn't put them high on the list.

For everyone, the reason I'm not so sure the ISV is the culprit, is that off-idle quite a ways, up to 2000 - 2200 RPM, I still have to feather the throttle to keep it from dropping to low RPM. Can a stuck ISV affect the throttle at 2000 RPM. If I push the throttle harder it will run steady at 2500 to 3000 or lower if under load. It accelerates normally and cruises fine. Also, that's why I hadn't considered the fuel pump or FPR to be a problem.

I swapped in another LH today - no change.

I hope to work on it tomorrow.

Bill Ball

Stan: Thanks for that link. I've seen stuck ISVs lower idle, but I've never seen one screw up off-idle behavior as I describe above. It's hard to hold the RPMs steady at 2000 RPM in neutral. So, it's much more than a low idle.

That is one nasty looking ISV in that thread. And I can see that you went all over the place looking at other possibilities before you got to that. I'm headed down the same path looking at other things first. Let's see what the Spanner ISV test shows. I expect that if I don't hear the usual "CLICK" with the test, it is frozen.