intermittent stall driving me crazy (not DME relay)
 

So, for the past couple of months I've been experiencing an intermittent stall in my 924S ('88).

- Almost always happens when coming to a stop, sitting at a light, or leaving a light.
- Occasionally I will get a stumble or some clicking (sounds like it's coming from the right rear of the car but hard to tell) a few seconds before it stalls. I'm not sure if the clicking is related, as today is the first time I've noticed it. Feels like it's not getting fuel. Sometimes it "recovers" and drives fine, other times, it doesn't.
- When it does, everything goes dead - no dash lights, nothing, as far as I can tell.
- Will not restart immediately. Usually have to crank it 5-6 times over about 30 seconds before it will restart.
- Car starts up perfectly and immediately when cold, idles fine. This usually seems to happen after I've been driving for about 10 minutes. Before that, car drives perfectly.

what I've tried:

- Tried several DME relays and now have the Focus 9 solid-state relay. None of these have prevented the issue.
- Tried both of my DME's - one is stock, one was rebuilt and chipped by the guys at MaxHP.
- Injectors seem to be fine. In any case, doesn't feel like a single-cylinder misfire. I don't think that would kill everything.
- Replaced the fuel pump, fuel filter, and made sure the screen was clean.
- Checked for vac leaks, didn't find any of significance
- Rebuilt the throttle body and made sure to set the TPS correctly so it clicks. Car idles and runs better with the rebuilt TB (probably had some vac leak there) but no effiect on the stalling.
- Figured it must be the ignition switch, so I pulled it out and it was cracked. Just put a brand new one in today, figuring that was the problem. Nope. Almost stalled once, did stall once. Same symptoms, same result.

- I don't think it's idle air valve related, since it will happen even if I manually hold the throttle open at higher revs while sitting there or driving.
- No sign of any blown fuses. Thought I might have a short in the fuel pump wiring, but I would imagine that would blow a fuse.

So, the last thing I can think of that may cause me to not be getting fuel is either the pump isn't getting power, or not getting ground. So I'm going to trace the wiring from the pump and see. Does the pump ground to the chassis? If so, anyone know WHERE this is? When I replaced the pump the ring terminals pretty much broke off in my hand when I unbolted them, so it has brand-new ring terminals.

Car does not have any alarm stuff, and never did, as far as i know.

Not smelling any wiring burning or anything that would suggest a short.

In any case, driving me crazy. Car drives great until just before it totally dies. So any other suggestions? Could it be a bad coil? But why would that only make the car die at low RPMs/no load?

Did you run a fuel pressure test: Fuel Pressure - Checking

I have not run one yet but I do have a pressure tester. I guess I'm kind of failing to see why the car would run fine up until the moment of stalling if there was something wrong with the fuel pressure. Fuel pressure was low I would imagine it would manifest itself more when the engine is under load or I am on the throttle.

The FPR and the FPD are controlled by vacuum. How exactly I can't immediately explain. The FPR or FPD could be bad, or the vacuum could also not be working correctly. This is what I am thinking.

Okay well I just checked it and there's definitely something wrong there. With the first test with the fuel pump jumped the readings were about right, at 38 or so (so within the 36+- 3 specified). But with the engine running, the reading stayed pretty much exactly the same, when they are supposed to be down near 29. So maybe injectors are just being over driven and shutting themselves down. Guess I will get a new fuel pressure regulator and see if that works....thx

Injectors cannot be over-driven as at full-throttle, they should have full 38psi anyway. These clues point to incorrect fuel-pressure at idle... which can be caused by multiple issues. At this point, we don't know if FPR is bad or if it's not receiving proper vacuum signal. So... we need to test both.

1. vacuum-leak anywhere after the AFM or throttle-body will throw things off. Measure actual intake-manifold vacuum with vacuum gauge. Check http://hfqpdb.com for coupons.

2. use vacuum-T fitting and measure vacuum at FPR and FPD. What is this vacuum number at idle?

3. Disconnect coil-to-distributor wire to disable ignition. Pull fuel-rail with all injectors connected. Place little cups underneath each injector and crank engine for 15-seconds. Inspect each cup to ensure that all injectors are spraying identical amounts of fuel. If large variation, rotate injectors between connectors to see if difference moves with injectors or stays same.

4. measure AFM voltage at idle. What is this number?

Interesting.....curious then as to why specs call for 29psi at idle as the correct spec ....at least according to Clark's. This is an 8V engine, by the way, so lower pressures than 16V.

If I had a significant vac leak, wouldn't the car idle poorly (it idles fine, except right when it dies) in general, and wouldn't I have reduced or absent power brake boost? (brakes feel normal).

I pretty much went over the vac lines closely not too long ago, even securing all the elbows with small zipties. But will give them another check.

If I unplug the vac line from the FPS while the car is running at idle, what *should* happen?

Can your eyes see air-molecules flowing? Or see electrons moving in wires? Or petrol molecules moving in fuel-rail? Without superhero powers, we need to use instruments to measure intake-manifold pressure and voltages and fuel-pressure. I’ll explain when i get to keyboard how FPR works and what causes it to not work properly.

try running a test wire directly from battery + to the + terminal on the ignition coil.
try for a few days (disconnect when you park the car) and see if the issues persists.

i had random stalling for a while and the black wire from ignition switch to coil + had a burnt socket in the central electric panel making spotty contact...i ran a new wire from switch direct to coil and no problem anymore.

smoke test

Really no need to be a dick about it. no place did I say I was going to visually check them, they've been checked for leaks at 5psi using compressor air and soap solution. At the time the only notable leak was at the throttle body itself, and that has since been rebuilt with no change to the behavior of the car.


In any case with a vacuum test I'm getting 40 vac downstream of the TB at idle, but zero at the upstream fitting before the butterfly at idle (with vacuum increasing at that port as I open the throttle further). As previously noted at idle my fuel pressure at the regulator is about 39, while Clarks listing the specs as being 29 at idle. This would seem to indicate that the regulator is not opening enough at idle to release fuel back to the return line . My question was only whether there should be any vacuum on the fuel pressure regulator at idle, or only under throttle.

there is no vacuum ahead of the butterfly.
after the butterfly you should see something like 15-20 inHg of vacuum.

there should always be some amount of vacuum on the FPR until you get to near wide-open throttle.

fuel pump on/engine off (or at WOT) fuel pressure should be around 37 psi give or take.
engine running at idle fuel pressure should be about 29-30 psi.

Will give that a try.

To add to the testing. If I plug the vacuum line from the fpr and damper into the port at the back end of the throttle body, fuel pressure drops to 29 just like it's supposed to be at idle. So essentially I know I'm just getting no vacuum forward of the throttle body (i.e. the jboot)

And some additional information. I just idol the car in my driveway for about 10 minutes. Idled just fine the whole time. Fuel pressure showing 39 the whole time. After almost exactly 10 minutes, the car just totally shut down while just idling. Fuel pressure is showing 39 still after the shutdown. So I'm definitely getting sufficient fuel pressure, and that is not causing the car to shut off. So it's looking more and more like something electrical.

After sitting for about a minute while I typed that, the car started right up again easily and is once again idling fine, and then stalled again about 2 minutes later. so it almost seems as if something is overheating in the electrical system and causing everything to shut down.And once it cools that works again but then shuts down once it gets hot again.

I think the next test I'm going to hook a multimeter to the coil power and run the car again and see if it holds steady.

the FPR/FPD are supposed to be hooked to the vac port after the butterfly.

the port ahead of the butterfly is for the evaporative emissions system.

Interesting. This is the way it has been set up for the entire two years I've owned this car, but now that I look at the vacuum routing diagram under the hood I am betting the previous owners switched them since the diagram is upside down relative to the engine itself. Or I guess it's also possible I switched them while doing something to the car at some point.

Okay I'm switching them now. If it turns out to be something this stupid that would be great I guess lol. Thanks.

Edit: ran the car at idle and it once again died after about 10 minutes.

So I guess that was not the problem.

The coil could be dying, check for a strong spark with a spark checker when it won't start. If you do not have a spark checker make sure the spark can jump 10mm. A coil getting weak when warm is common.

You can also try some starting fluid when it's stalled to see if that makes it catch. That won't rule out a spark problem completely, but it'll be an interesting tidbit.

When it won't start, what do the plugs look like Are they wet or dry? Smell like gas? Clean enough?

If the plugs are wet and the spark is OK it might be the DME temp sender.
-Joel

Well after all this it looks like it had something to do with the reference sensor plug. I replace both sensors as well as the wires all the way to the DME just a couple months ago, and the car ran perfectly after that. So I had it running just now and was going to unplug the O2 sensor to see if that did anything. But when I undid the cable ties that also holds the reference sensor plug, I went to move it out of the way the car died immediately. I turned it back on and tried it again and the same thing happened. The plugs are feared tight no play and like I said everything is brand new there. Anyhow then I unplugged it both of them and plug them back in, and now the car has been idling nice for almost 20 minutes, cycling the fan on etc. So I guess it's just a bad connection or something caused by the way I had to wires tied down or something. Maybe they were pulling the plug apart just enough to stall the car in certain occasions. So looks like the problem is solved, at least for now.

Thanks for all the help and suggestions! Turns out to be something dumb, of course.

I’d like to know more about this - was the plug simply not pushed in all the way? Is there some obstruction that makes plugging this in unintuitive?