Stutter & misfire troubleshooting

[Panzer909]

So now that the belt, seals, cooling, and motor mounts jobs are out of the way, I'm chasing down a constant stutter from idle thorughout the RPM band, along with a misfire under heavy load.

Changed the usual suspects: plugs, cap & rotor to no change. Haha yes the cam belt is perfectly aligned. The car "functionally" drives, but at idle it sounds like a galloping horse near the exhaust, and when driving at all RPMs it feels (and sounds) like it has a rev limiter and I'm an F1 driver going down the pit lane during a race.

Here's some interesting discoveries:
1. After only driving about 50mi or so on the new plugs, they're black filthy (pic included) so running rich(?)
2. Disconnected the O2 sensor for a short drive and no change although that doesn't suprise me.
3. Pulled the wires on at a time with the car running and little if any change when I pulled wire #3. But there was voltage within range at each wire..
4. Did the screwdriver test at each injector and they seem okay.
5. Pulled the cap again and found a crack. Nice! Must have over-tightened it after the belt job. I did read that an uneven surface on the S cars causes this sometimes. But remember that the car was doing this before I changed to this cap.



And the new cap:


Obviously could be several things and I'm kicking myself about the cap. Bought a new fuel filter but didn't put it on yet. Anyone ever experience similar symptoms as I continue to dig into this thing?

[Gawernator]

Fuel injectors?

[bell]

Double check your plug wires, make sure firing order is correct.
Even if you think they are right.....check them again against rotor position and the tdc mark on the crank.

[rhett]

Also check the ignition module, I have had problems in that area..bad connections cause lumpy idle, misfire, etc. Just a thought. Clean the contacts and WD40 them, it might help.

[Gawernator]

Where is that??

[rhett]

Originally Posted by Gawernator

Where is that??

Its only on the 16V models. A small plastic module on the inner wing mounted near the rad header tank behind the headlight. It gets forgotten, but it is a bit exposed to the elements as water can get in there.

[guru944]

Panzer909;

Well, My S was suffering from the very same thing. The car would be fine after a cold started. However once it got warm the needle was below mid-mark on gauge, it would sputter and back fire from idle trough to +6000 rpm. Checked the usual vacumm, plug gap, etc. and also noticed my fuel pressure would change (have a gauge and adjustable FP) and suspected fuel pump for some time now.
I opted to change the eventually replace the DME temperature sensor (next) and upgrade to a HV Bosch 040 fuel pump after much fustration. Flushed the fuel tank, checked the in-tank filter for dirt or clogging (It was clear, like new)

WOW! What a drastic change in personality. Dropped the fuel pressure from 3.5bar to 3 bar (44psi) The exhaust tone got deeper, Idle was steady and rock solid. Still want to replace my iridium plugs (2yrs old) and ignition wires....

DME Temp sensor install pending...

Will post install & updated pics soon.....

Thank you Dave at Lindsey Racing for the Excellent Tech support and products...

PM Me if needed.

Blessings...........

[Panzer909]

Sorry for the late response guys, I was busy with family/easter stuff.

Rhett, will do on the module - mine is looking a little funky from the outside so I'll clean it out tomorrow.

Guru944, I already ordered a temp sensor along with a (2nd) new cap before the weekend, so we'll see what happens. Upon initial startup, the car does in fact make a small whiny buzzing noise & I don't remember it sounding like that before. I was assuming it was the new belts. What would you recommend to test the fuel pump if you don't have a fancy guage?

I also found out that my battery won't hold a charge after a few days, so it's on the way out. I read on the 928 board that this had similar effects for a guy over there. Doubt it's the culprit, but it will get replaced anyways. Also going to unplug & further inspect the #3 injector since there was no idle change when pulling that ignition wire.

Happy Easter!

[Gawernator]

Well, the F1 with the rev limiter still sounds pretty cool

[Panzer909]

Ha but when you're trying to drive the car, not so much

[guru944]

Panzer909:

See pics below & Enjoy... Later

[mytrplseven]

Here's my take, FWIW: check plug connections to the TPS, the AFM, the firing order on the plug wires, the DME temp sensor (cheap replacement), all the vacuum line connections (including the one under the J-boot and get a noid light at the parts store and check for impulses into the injectors.

[Panzer909]

Update:

So my replacement cap and temp sensor came in today. Put those both on and.....no change. I did unplug the suspect #3 injector with the car running in the garage and there was no change in the idle (just like when I unplug the ignition wire for that same cylinder.) So I might have to yank that injector. I did do the screwdriver test to it and it was clacking away just like the others. Hmm.

[968 GUY]

Teflon tape on an AN fitting?????????????

[guru944]

968 guy;
Yes Teflon will protect the threads from ionization. and potentially provide a better tighter seal....
Liquid teflon works best.
Later...

[mytrplseven]

I still recommend a "noid" light instead of a screwdriver test. It's cheap and really shows when the injector is getting it's impulses. The injectors can be rebuilt perfectly by WitchHunter in Bellingham, WA. It's only $100 shipped. Did you check the firing order on the ignition wires? Have you looked at the connectors for the TPS and the AFM? Try moving them a little while the engine is running to see if it changes. By the way, trying to do an engine check with plugs that are still covered with "junk" will mask anything you try. Also, make sure the distributor cap is seated properly on the distributor when locking it down.

[Ky914Newbie]

With it running so rich, and you have looked at a lot of the obvious. I would recommend confirming the injectors are the correct size. I have seen several people with mismatched injectors recently.

I may have missed it, but I assume vacuum leaks have all been chased down and resolved. If so, did it pass a pressure test?

[mytrplseven]

If you haven't checked for all of the possible vacuum leaks, Ky914,s right. I bought a 2" pvc fitting and screwed an air fitting into it and inserted it into the J boot. Applied around 10 psi of air from compressor and sprayed the entire engine with soapy water solution. Bubbles came out of EVERY vacuum leak, including the tiny rubber O rings around the throttle body shaft. There's also a couple of other areas that are harder to see unless you know where to look: the cannister next to the battery, the T fitting under the J boot and the idle stabilization valve under the intake manifold.

[Gawernator]

Instead of looking, I just replaced all the hoses/O ring seals and vacuum line lol

[Panzer909]

I ordered a Bosch noid light so it should be here shortly. I agree that the screwdriver test doesn't say much. I'm almost convinced that it's either the #3 injector itself or the DME isn't getting a proper signal to that cylinder. The noid should tell me a lot more as soon as it gets here.

As far as vacuum leaks goes, I haven't rigged up an air fitting yet, but you guys are sharp with that pvc/compressor idea. Nice! that will be next if the suspected injector fault doesn't fix everything. Also, I REALLY dread having to remove the TPS and/or intake. On the S those are a real pita.
Updates soon