I'm guessing fuel pump.

Just to confirm, when you are turning to key to the Start position, the starter is indeed turning and turning?

Replacing the pump isn't exactly hard, but I'd want to know it's the problem before just pulling it. If the pump is suspected, I'd start by putting a meter at the pump itself, and be sure you have good power and ground. If yes, then I'd disconnect an injector feed, and visually verify delivery of fuel. (Cover it with a rag, and don't smoke while you're doing it...)

 

From what you've described, I would not suspect that the fuel pump is the problem. (But you can test that it works by either jumpering the pump at the relay panel with a length of wire, or opening up your airbox and pushing down gently on the air metering plate with the ignition switch on.) The fact that your motor cut out momentarily and came back up in not usually indicative of a fuel pressure issues. What you have sounds more like an ignition fault problem, honestly. I had a similar intermittent fault that built up slowly over time, exacerbated by heat soak and high oil temps. Turned out my oil return line had gotten nestled against the wire from the crank speed sensor and cooked it. That is the only major system component shared by both front and rear bank ignition systems, so it caused both to fail whenever it heated up past a certain point.

I would look closely at the wiring harness that runs to/from the crank speed and TDC sensors and your ignition modules. You can remove those sensors and bench test their resistance cold vice warm vice hot (they should be in the 600-1000 ohm range). Apply a heat gun and see if the resistance gets excessive as you warm them. Needless to say, you should also verify you have good, solid ground everywhere in the ignition system. The coil packs are grounded locally, and I think the ignition boxes are grounded via the diagnostic plug (look up inside the left flying buttress from inside the wheel-well) but there may be another point I don't remember.

Steve Magnussen here helped me trouble-shoot this issue on my car, so hopefully he will chime in.

Good luck.

 

It's also not clear to me if the starter is spinning in your description. If it was, and if you had let the engine get warm, it could be a warm-start issue, but that should go away after the engine cools back down (and in many cases if you give the throttle pedal a healthy press while cranking the starter, that will overcome the problem). Once you determine if it was a warm-start problem, there are some things to check, starting with simple ones and increasing in complexity to maybe needing a pressure tester to diagnose.

I just replaced the fuel pump and accumulator on my 328 (old accumulator was leaking across diaphragm). I had no reason to doubt the old fuel pump but the banjo connection was leaking and I figured I may as well replace the pump while I was in there for the accumulator (while I had the tanks drained I did a whole bunch of loosely related work, classic "while you're in there" overkill).

Anyway, one odd side effect of this work is that the engine previously had an intermittent stumble at idle. Never was a driveability issue, but I'd hear it every 5-10 seconds, almost like a pause or disruption in the rhythm of the idle. I was pretty sure it was a sign I needed new injectors. Well, after the pump and accumulator were replaced, that stumble is gone. I don't have a good explanation for it and would have never expected that. I'd say it was psychological but it was an unexpected thing that I wasn't even trying to fix during this round of work.

I know the accumulator has some secondary function to reduce noise and vibration in the fuel system (primary function is to maintain a background pressure for a short time after the engine shuts off) so it's possible that was related but I doubt it. I had also found ethanol damage in the hose connections from the tank to pump and it's possible they were letting air get drawn into the system since the hose is under suction from the inlet side of the fuel pump. It's possible the new hose and clamps would have fixed that.

None of that was on my radar initially, I was just in there to replace accumulator and pump and some hoses. But something I did cured that intermittent stumble at idle.

 

Remove the big hose from the intake plenum. Spray in a full five seconds of starter fluid. Connect the hose back up. Start it, if it fires and runs for a few seconds, then you have confirmed lack of fuel to be your problem. Chase it down from there.

 

I think I am in the middle of the exact same problem. I went from a misfire and stumble to now the car won’t even fire. Subscribing.

 

Hard to make a diagnosis on flow rate at the injector while cranking since the metering rate is fairly low at that condition.

A better fuel pump test (I think) would be to disconnect the return line going to the right side tank and see if that is flowing (you'd need to route it to a jerry can with an extra section of tubing). It should be a fairly substantial flow at a cranking condition, since most of the fuel pump's output would be bypassing the metering device and going right back to the tank.

A blocked up fuel filter could restrict the flow and that might be something else to check. I'd expect the pump to be making a lot of noise and vibration if that were the case. When was the last time the filter was replaced?

 

Has been long since you replaced your spark plugs? This is an unusual thing, but it happened in an old MB. Same symptoms, checked pump, filters, electric systems, replace distributor covers….. made no sense.
It was all due to spark plugs.

Fuel pump relay?

 

If you have a CIS fuel pressure test kit, you can find out what the system pressure is at the WUR. All your fuel pump questions would be answered then.

 

I think drips are hard to decipher at this low throttle setting. In reality the system runs at around 70psi but there is a pressure drop through the orifice(s) of the metering unit and then a bigger pressure drop at the injector orifice (that one is much more restrictive). I’d expect more than drips out of the injector supply line but it’s an unknown to me at low throttle settings. Really you want to be testing or sampling somewhere else in the fuel system where the pump’s 70psi hasn’t yet been restricted.

 

This isn't a valid test because the airflow metering plate is not in a very deflected position (since the engine is drawing in very little air with the low RPM of starter motor cranking). You would have to also have to manually hold the airflow metering plate in about its 1000 RPM idle position to make a measurement to see if the amount of fuel delivered was in the 6~7 cm^3/min spec range of idle at each injector. However, the fact that you got anything out is a positive sign that the fuel pump is at least doing something. A more significant tests would be to measure the return flow going back to the tank with the fuel pump running as s129 suggested -- as this would also confirm/deny if the fuel filter is blocked, or not -- should be more than 1 liter/30 seconds (see page D34 in the Mondial8/QV WSM). Better still would be to get the CIS pressure gauge as Mitchell suggested.

To run the fuel pump manually either:

1. Unplug the safety switch on the airflow metering unit and turn the key "on":

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or

2. Remove the fuel pump relay and use a jumper wire to connect the female 30 terminal in the relay socket to the female 87 terminal in the relay socket:

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The fuel pump should run (even with the key "off").

However, you should really confirm/deny spark during starter motor cranking before worrying about fuel -- you can get a very inexpensive "spark tester" at an auto parks store or online (basically a neon bulb that will flash if brought into close proximity to a spark plug wire and a spark is occurring) if you don't have access to a timing light.

What version 1985 308QV? US or euro?

 

That bit occurred to me very late last night; my apologies for omitting that - hope it didn't lead anyone astray or waste too much time. :/

 

This is why a spark tester is more handy to check for absence/presence . Seriously, on my ex-308, I had to resort to running a temporary +12V lead from the positive battery terminal all the way back to the engine bay to use my timing light.

 

I had an intermittent issue with the fuel pump cutting out when warm - it was an old ceramic festoon style fuse on a bypass inline fuse for the birdman fusebox. Rewired it with the glass fuse - Voila. BUT in the diagnosis replaced the w fuel pump, accumulator and anything connected. the fuse made it bulletproof.

 

A leaky accumulator diaphragm is one of the prime causes of warm start issues, and it could also cause a cold start to take longer than normal. Seeing fuel in the accumulator vent line is a sure sign the diaphragm is leaking, so the accumulator should be replaced. That is Bosch part # 0438170004. And you may as well do the pump too, Bosch part # 0580464151 (Bosch USA # 69532 is the whole kit). The accumulator is the same part going all the way back to OEM, but the fuel pump is a crossmatch since the original pump is no longer available.

My filter was replaced by the previous owner's mechanic a couple years ago, so I left it alone, but if yours is original or has a lot of miles on it, you may as well bite the bullet. That is Bosch part # 0450905601.

While you have the tanks drained, it's a good time to replace the connection pipe sleeves if they are original. That is a pretty easy job, although you'll need to drain a bit more fuel that is sitting in the pipe, so keep that in mind.