jpmurphy99

Hi all,

This is my first post but I've been spending a lot of time lately absorbing the great info on this forum. The information here has been immensely helpful to me in finding a 928 and fixing some of the issues that came with it. Unfortunately, I've hit a bit of a wall with the car. When I bought the car, the PO told me that it had been stalling occasionally upon start. I never could reproduce it during test drives but it happened to me a couple of times after purchase and I decided it had to be electrical. I went through Wally Plumley's electrical PM procedures and the car hasn't stalled on me since - thank you Wally.

Unfortunately, it now has a surging acceleration problem that it didn't have before. Basically, in any gear, the car will start surging or have jerky acceleration when I hit 2700 RPM. It also has trouble finding idle - particularly when I step on the clutch while approaching a stop sign or light. It will drop to well below 1000 RPMs before it over-corrects. I figured I must have inadvertently unplugged a vacuum line, so I created a vacuum troubleshooting device based on ones I've seen on the forum and went hunting for leaks. I immediately found a big leak from a shot FI seal. I replaced that seal and ran the test again. Unfortunately, I can't create any pressure at all and can't hear or find a leak. I'm also less convinced that my surging issue is vacuum related since I'm betting that FI seal was shot when I bought the car. So now I've lost my steam. I'm ready to take it to a pro but figured I'd seek some advice here first. Any ideas on what I could have done that would have caused this kind of problem? The car is an 86.5 5Spd. Sorry for making my first post a novel and thanks in advance for your advice.

Speedtoys

Not that I can help, what year, and transmission?

jpmurphy99

I thought I'd put this in my personal info, but I'm having as much luck with that as I am the car.

Speedtoys

Might be, but we dont see it -here-.

Mike Frye

Hey JP,

Welcome and congrats on landing an 86.5 5-speed. It's about the perfect car IMO. A great platform that can be developed into an incredible car.

OK, so for the vacuum issues you're having, you should be able to build up a little bit of pressure in the intake if you built one of the jigs you saw on here. If you can't figure out where it's leaking from and can't build up any pressure then you'll have to do it in pieces.

It isn't easy, but you can actually test just the lower air guide and throttle body this way only with it in the car:


If you want to test the whole system know that there is an O-ring to seal the dipstick handle as well as the bottom of the dipstick tube (where it goes into the lower block, that's where I found a bad leak). There is an O-ring in the oil filler cap that can be really brittle and leaking.

Vacuum leaks can be a major PITA and can have an effect on idle (surging or high idle). I'd definitely try to figure out how to get them all fixed. They'll rob HP and kill your enjoyment of the car.

Having said all that, I'm not convinced that your problem is an air leak.

I would verify that the idle contact is resetting and that the timing is reflecting that (switch may meter OK, but if there's a break in the wire somewhere or loose connection the LH won't get the signal). If you check it with a timing gun I think idle should be about 10 deg. BTD, 20 is just off idle, then it jumps up again when the WOT is engaged. Check it cold AND fully warm. I had a problem where the idle switch worked fine cold in the garage, but when fully warm it wouldn't reset.

So first make sure you can hear the idle contact when you manually pull the throttle cable with the car off. Then start it and check the timing and do the same thing. It should reset to 10 deg. when the idle contact resets. If so, the LH is getting the idle contact signal and responding correctly.

Finally, I think you might want to check/replace your O2 sensor. Mine was doing weird things to my idle as well.

For the surge at 2700, does it do it in all gears? If so, you WOT switch might be engaging at that point. Not sure what that one could be.



One last thing: You said you found a 'shot FI seal'. Do you mean the O-ring for one fuel injector was leaking? You said you replaced 'that seal' and ran the test again. If you had the fuel rail out, I'm hoping you meant that you replaced all of the seals on that side at least. If one seal was bad, most likely the rest won't be far behind. Sounds like a top-end refresh should be on your winter plans, but if not I'd replace ALL of the Fuel injector O-rings. Doing one is at a time is just asking for headaches.

Good luck and keep us posted!

Darklands

Great helpful post Mike.

@jp,
I have also a S 3 5speed and it was a never ending story before I had a perfect running engine.
Have you vacuum at the computers and the pressure dampers?
Is your idle valve working?
Check the rubberparts and lines from the heads to the oilfiller neck and from the TB to the oil filler neck.
The rubber there is hard as plastic and break very easy.
Check the temperature sensor II. It´s 57 € from porsche,maybe it´s a good idea to buy a new one.

Here´s the advice for tune up a 86.5 from Ken at liftbars.com.

Minimum to idle properly:
O2 sensor
Spark plugs
10 upper intake hoses
ICV (cleaning won't help)
CO/idle speed adjustment (Blink'r)

And often needs:
TPS
TPS harness
Long ICV hose to 'T'
Temp II sensor (coolant)
Rebuilt MAF, MAF o-rings

For general running:
Fuel filter
Fan clutch
Plug wires
Front crank seal
PKsn'r, timing belt
6 cam end seals ('85)
Distributor rotor, cap
Crank position sensor
Jump post cap + cover
Temp I sensor (airbox)
Water pump, new WP bolts
Thermostat, o-ring, rear seal
Cam/oil pump gears (if coating worn)
Cam timing check/adjustment (32V'r)
Flexplate release, PKlamp (auto)
Fuel pressure regulator return hose (bulk 7mm ID)
Air filter (throw away K&N, clean dirt from lower airbox, MAF/TB housing)

I hope it helps.

jeff spahn

I was going to suggest the ISV but you beat me to it. I think you can get these out on an 86.5 without removing the intake. hard but possible.

shmark

Just wanted to say this is a great thread for concentrated information on troubleshooting any running issues. Bookmarked.

Mrmerlin

JP to add info to your signature ,
go to top of page,
click on user CP ,
then click on edit signature add any info about your car, year, model, trans, color, where you live

Kiln_Red

Well, if it isn't in your capabilities to fit a gauge to the intake, you could also try searching for the leak by using starting fluid. Or have you already tried this? From the symptoms you mention, it definitely sounds like a vacuum leak. Can it be heard just by standing in front of the engine or is it not that obvious?

Mrmerlin

WARNING be very careful spraying any kind of flammable liquids on a running 928 engine the faulty spark plug wires may cause an explosion

KLVA

Welcome to the Tank, my friend. You should check all the posts and info and pics at the DIY section at the bottom of the RL forum and you will find Dwayne's Garage and miles and miles of detailed info and procedures about your car. He is also in California and can help you as needed...just passing along a resource...Good Luck and Cheers....Kevin

Kiln_Red

Soap & water gets the job done too.

jpmurphy99

Thanks Mike - I was actually wondering how to build just that kind of rig to test the intake. And thanks for the pic, it's very helpful. Is the idle switch the same as the TPS? I was doing some research last night and some similar threads were pointing in that direction. I plan on checking that one tonight as the harness leading to the TPS (and many other parts of the car) is crumbling. The surging or jerking does happen in all gears at 2700RPM. The feeling is probably best described as jerky acceleration.

As for the FI, it was the o-ring at the bottom that was bad. Basically, there was a piece of it that had been sliced cleanly off. Not sure how. I didn't replace the other o-rings, but they all looked fine.

I'll test the TPS tonight per the WSM and post what I find. I actually don't have a timing gun, so that test may have to wait a day or two.

Mike Frye

Yes, when I mentioned the idle switch, I was referring to one half of the inappropriately named 'TPS'. The 'Throttle position switch' is really just a set of contacts that have three states (for the purpose of this discussion).
1: idle
2: cruise
3: WOT

The LH determines which fuel map to use based on this. If the idle is dropping down really low but the idle switch isn't reset, it's going to try to fix it with the cruise map rather than the idle fuel map and the cruise map doesn't expect it to come down below 750 or so, so it's going to bounce.

Good luck with it.