F18Rep

I think you're going to end up looking at the TPS but the fuel pressure test is a great idea. To have good (steady) fuel pressure the sensors, DME, relay, pump and filter are all verified and besides, hooking up a fuel pressure tester is literally a 1 minute job.

An oil filled TPS will give some of the symptoms you have described (not certain but, I'm thinking you have a turbo) TPS failures are common on the turbos. They are pretty easy to test and with some luck they can be popped open and hosed down with carb cleaner. If you get passed that, the AFM barn door would be suspect and it too can be tested pretty easily.... just some ideas...Bruce

Ronin-951

Cannot overstress how important the Grounds and the Vacuum lines are, ... and what a bear they are to troubleshoot. Tend to these no matter the outcome, as they are the foundation of engine operation. Think your clear with the reference sensors, btw, you can see them at work when cranking by watching the Tach for a 1/16" bounce. Yes, the DME is known for some issues, there's some cold solder joint problems that develop that should not be attempted DIY, as well as some TPS issues that will void the core return value if you open it, both of these can be checked by sending them to '944online'. Then there's the ISV ...

YoungerThan944

Thanks for the input! My car is an '86 N/A, should I still be lookign at the TPS? I already "refreshed" my AFM by moving the resistor surface contactor, but it made no difference and looked pretty good in the first place.

I'll hook up a fuel pressure tester tonight and fire it up - could pressure being too high cause the DME to cut spark or fuel?
Edit: I just read a clark's garage saying a faulty FPR could cause my symptoms. That's tonight's test.

The 1/16" bounce is there. I live in a very damp climate so the rubber in my car is generally in good condition. That being said, I know I have one vacuum line that is cracked (on the back on the fuel pressure damper). It ran with it completely disconnected before I had this problem so I don't think it's the root cause.

As far as the ISV goes, could it cause the engine to die under power? When it first died I lost power under throttle at about 2500 RPM.

Ronin-951

Doubt the ISV has enough control to kill the motor under power at 2500 rpm, but the vacuum leak can effect about ten different systems and MUST be addressed before you can go further. Get a COMPLETE kit with new tee's and a new check valve, it's the only way to be sure and not have to go there again. Clean the grounds as you go, the block to firewall cable gets alot of abuse with every heat cycle.

YoungerThan944

I just tested the fuel pressure. 35 psi while the cylinders are firing, and 40 immediately after they stop. I also unplugged one injector with no success.

I'm hesitant to redo all my vacuum lines. What bugs me is that the car ran fine for two weeks after new S&R sensors were installed, and then died with the same symptoms it had in the first place. Could a bad ground somewhere be causing this? Does anyone know of any specific grounds to look for? I'll check the block to firewall cable shortly.

YoungerThan944

So I have an update. I was cleaning grounds (cleaned the main negative battery terminal) and I found a vacuum line fully unplugged way down in the back of the engine! Plugged it in, it fired right up. I let it idle for a bit, and started driving it down the block. The car made it half a block and it died the same way it has previously.

When I fired it up after it cooled down a bit, an ex-mechanic down the block held the throttle constant. The engine surged between 2000 and 3000 rpm while warming up without any consistent pattern. Eventually when it warmed up enough, the revs started dropping off and it unevenly slowed its way to stall. He wasn't able to suggest anything but taking a look at the DME.

When I fired it up again, I was back to where I was before - the engine turns over and gets to ~1000 rpm before cutting off immediately, even after letting it cool down. Why??? It's like something is getting worse every time it gets to temperature..

Does anyone in Vancouver have a spare DME I could plug in? Or is there a local service to test the DME?

mytrplseven

the Haynes repair manual for the 944 has a diagram in it with all the ground points. I'd try to find one and go from there. Good luck. This stuff is maddening.

YoungerThan944

Thanks, I'll go through a thorough search of grounds and vacuum lines. Took apart and resoldered the trouble areas in the DME today, that didn't fix it.

F18Rep

OK, here's one other idea, have you recently had the fuel filter or pump out? I had a wierd one once where I had replaced both and was seeing erratic operation, (hard to start when warm, and would die on me once it warmed up). I went through to the fuel gauge just like always but had good pressure - so dismissed fuel. Then after some scratching, decided I would bleed the fuel line. I used some clear plastic drain line and jumpered the new pump, and sure enough got air bubbles - after that all was good again.

I'm still not quite sure this makes sense because I had good pressure and would have thought the air would have just migrated out an injector pretty quickly but I'l take it as a win....Bruce

YoungerThan944

I did recently replace the fuel filter, I'll definitely give that a shot! Thanks for the suggestion.

YoungerThan944

I tried bleeding the fuel system to no avail. However, I noticed when I re-checked the pressure that the pressure dropped off quickly after the car stopped cranking over - apparently a sign of a bad check valve. Could this be related to my problem? Keep in mind that when I was able to get the car running the other day the RPM's were searching between 2-3000 at constant throttle.

harveyf

It's really a shame you don't have a known good spare DME to plug in as a test. My car did just what yours does ie. start, rev, and die. I swapped in another DME and that solved the problem

YoungerThan944

I'm tempted to take an expensive gamble and just pick a new one up. I'll take apart my DME again and resolder all the joints to see if that helps at all.
I'll also order a vacuum line kit tomorrow to eliminate that possible issue.

If that doesn't work, does anyone have a working DME I could buy off them? Or is there a cheap place to pick up a new/rebuilt one?

harveyf

There are several places on the web that sell the unit you need. Most require a core to exchange I think. Or look at the craigslist ads in Vancouver and call of few of them and offer to rent their DME. Sounds crazy but an enthusiast might go for it, just to help you see if that is the problem before you spend $250. Heck, someone may actually have a spare one. I've got 2 but I'm 3000 miles from you.

For me, it was after an engine rebuild and full rewire. So many places to go wrong. I could get the car to fire and catch but died within a few seconds. I used my Fluke meter to check the sensors and they seemed OK. I went to bed really frustrated. About 4 am I remembered that the PO has given me an extra DME. I went out and plugged it in and the car fired right up. Oh, what a feeling

YoungerThan944

AUGH. I think I may have found the source of my problem.

Last year when I was cleaning grounds I dropped one in the clutch bellhousing, and took everything apart. I lost both ground bolts on the bellhousing during the process, and replaced them with bolts I had lying around. One was regular steel and the other (battery negative ground) was zinc-plated. I thought nothing of it at the time, not knowing how sensitive electrical was. The car was stored for the winter.

The car was driven for roughly two weeks before I started running into these problems.

Could this be the source of my hellish troubleshooting? Where can I pick up good ground bolts?

I have a vacuum kit and used DME on the way, so these will be replaced before I try firing up.