mcpheatauto

I am looking for a bit of insight into the LH-Jetronic fuel injection fitted to my 1984 928 S2. Some background, the car was bought in 1984 and by late 1986 was parked up and never used again. I bought it 2 months ago and have being going through a process of resurrection. I had to fit a new fuel tank, pump, filter. All fluids drained. New cam belt and all other belts. Fuel injector rails and intake pipes all removed, tested, refitted with new seals. New spark plugs, caps, rotors. Good compression around the engine. After some churning I have got it fired up and have driven it. Now the problem. At cold start the engine always fires but idles low and slightly rough, around 500-600 rpm. If I goose it the engine revs freely but pops through the exhaust. When driving it it'll make it's way up to speed but is constantly popping unless, and here's the crunch, I floor it, in which case it lifts up its skirts and flies down the road, firing on all cylinders and smooth as silk. The throttle switch is clearly working and telling the system to do what it should so the injectors must also be working, fuel pressure should be good too. That must mean something in the mixture control is reading wrong at anything but full throttle. Today I replaced the temp II sensor to no avail. I also pulled the aux air valve and tested it with a hot air gun, it's certainly not stuck. I'm looking for clues and wondered if someone on here might have something to add. 31 years off the road has created an interesting car, I'd love to get it running just right, any ideas?

ROG100

Either injectors or MAF

Wisconsin Joe

Have you checked the MAF?

Mrmerlin

Replace the MAF and set the running mixture OHMS will be around 175, new MAF will be set at 375 to 380 ohms

verify the injectors are correct for the year ,
verify the distributor is correctly timed, both rotors should be centered on hash marks at TDC
Idle switch should click when the throttle cable is opened
https://rennlist.com/forums/928-foru...light=euro+MAF

mcpheatauto

I haven't checked the MAF because I don't yet know how to. I did check the throttle position switch at the switch and also through the loom from the ECU plug, works for idle and full throttle. It's a stick shift, by the way. I'l; check the rotor positions in the distributors but think they must be ok because of how it runs on full throttle.

Wisconsin Joe

Try this:

http://www.jdsporsche.com/main.html

Mrmerlin

if the MAF has never been refreshed then just have it done now, they do go out of calibration over time and the old circuit boards will be replaced with newer units

SteveG

You're in good hands I think. I can't help, but I'm (we are) intrigued by a car that was only driven for its first 2 years of life. I'm guessing it has about 20K miles (12.5K km?) on it? Sounds like a keeper. We're nosy.

mcpheatauto

Thanks Wisconsin Joe for pointing out JDS, I called John Speake today and he couldn't have been more helpful, what a nice guy. He wasn't so sure about the MAF being faulty because of the low mileage (30k). I checked the fuel pressure at the rail (30lbs), vacuum pressure at the damper and the ECU (15-18 in Hg). I tried to start it with the MAF disconnected and it would fire but nor idle and not rev above 1000, kind of implying there is fault elsewhere. Hooked back up and run up the road it still displays the problem, I wanted to do this to check the plugs. #8 came out wet so I've ordered wires, really can't excuse not having done that job yet. If that doesn't fix it I'll send the MAF and both ECUs to JDS and have them checked so I can rule them out at least. One step forward etc. For those that are curious to see the car I have put some photos of the car on my profile and there are more on flickr ( ). Thanks all for your help so far.

GT6ixer

Awesome interior! Looks like the pasha has held up ok.

A couple of suggestions. Have you replaced all of the little vacuum lines and their connectors? On my '84 they visibly looked good but would not hold vacuum. A leak here could cause a lumpy idle. Cheap but a bit fiddly to replace. Here is a video I made that catalogs their routing which can be confusing to trace.

Wisconsin Joe

Oh my. That looks like it has a ton of potential, but is going to take some time, effort and money to get it nice.

Love the Pascha.

Glad I could point you in a good direction. I don't pretend to know a whole lot about the MAF or the computers, but I do know who to ask.
By all accounts, John Speake is a very nice and knowledgeable fellow.
And JDS is at least in the same country as you.

Hilton

Cool car.

You're on the right track.. Start with making sure you've got spark on all cylinders, no vacuum leaks, and then get the CO adjusted at idle to the 1% mark per factory manual (most garages that can do an MOT will have an exhaust analyzer).

MAF's do go leaner as they age, but there's a fair bit of adjustment on the potentiometer to compensate, you just need to know what the correct adjustement point is at, hence the idle calibration.

mcpheatauto

An update. I got a new set of plug wires from design911 (nice set that were dead easy, all the right length and with all the fittings attached) and it still had a problem. I was somewhat pleased, didn't think it was the fault but glad I replaced them anyway. Nothing for it but to pull the MAF and ECUs and send them to JDS. John got back to me yesterday (working on a Saturday - I certainly wasn't), the MAF was a bit out on calibration, LH ECU is perfect but the EZ-F is faulty and pins down the misfire. Should have more news this week, might even have it back. I can't wait, even running like a dog it's one of the nicest cars I've driven.

mcpheatauto

I had a call from John at JDS today. The fault with the EZ-F ECU was one he hadn't seen before and it took a while to track it down to a faulty Integrated Circuit. He managed to find a replacement component and got it fixed. He's got an S2 and had fitted my unit to his car when it was faulty and did the same when it was fixed so it's definitely good. It'll be here tomorrow.

Wisconsin Joe

Good to hear.