94 Explorer shakes then stalls - error 543

[Dennis94XLT]

Today I started my truck after it had been sitting for about an hour. The engine shook hard then stalled. I started it again and it started and ran fine. I went about my business and at the next stop, same thing only this time it started and shook like crazy but didn't stall. I tried reving it and it acted like it was running on half the cylinders. Suddenly after about 15 seconds it snapped out of it and ran normally. It did it again when I got home - waited then restarted it. I am getting a 543 code with the Actron test device and the book says this code means

" Fuel Pump (FP) circuit open connection - Electronic Control Assembly (ECA) to battery."

I am pretty good at working on this truck but I dont' know where to start with this problem and I'm afraid I'm about to get stuck somewhere. Anyone have any suggestions or same problem with a cure?

I swapped the fuel pump relay with the AC relay (same part number) just in case that was the problem - but I'm wondering if someone more serious is going on - like an electronic control module failure. So far it has only happened at startup but I'd sure hate to be at a red light and have this happen.

Thanks everone,

Dennis

Thanks

UPDATE: Fuel Pump Relay was swapped with the ABS relay - and suddenly I was getting ABS power up errors. I replaced both solenoids with new and the problem(s) when away.

 

[MrShorty]

Was it a KOEO code or a memory code or both? Basically it means that the computer is seeing a problem in the fuel pump circuit. If it's consistent enough, it should be easy to track down with a wiring diagram and a voltmeter. It could be a little irritating if it isn't consistent enough.

 

[Dennis94XLT]

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[budwich]

I don't think the fuel pump is regulated electronically.... it is on with your key and is only turned off if you don't start (by the pcm). The regulation is done at / near the engine at an in-line regulator. In the end, you either get pressure or you don't.... there isn't any "turn up" thing.

I suspect that you might have solved your problem by swapping the relay... hopefully.

 

[Dennis94XLT]

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[MrShorty]

Fuel pump operation is controlled by the PCM according to the following simple algorithm:

1) Run pump for a couple of seconds immediately after the key is turned on.
2) Then wait for the engine to be started (specifically, wait for the "tach" or "PIP" signal from the crankshaft/camshaft postion sensor).
3) While receiving tach signal, pump is on.

The computer doesn't do anything to the pump to control demand according to fuel pressure. Fuel pressure is regulated by a purely mechanical back pressure regulator on the fuel rail. Your symptoms and the KOEO/CM 543 suggest that the fuel pump may be running intermittently rather than continuously. A bad fuel pump relay could be a culprit, or any other bad component/wire/connection in the fuel pump circuit.

 

[budwich]

As Mr Shorty indicated, the pump is not cycled.... if it was, then you would probably have real problems in the long term with keeping it functional... things like that don't like to be turned on and off repetitively. The "extra" fuel that is not required is cycled back to the gas tank by bypassing the regulator.

As far as the relay goes, it is a good candidate for intermitent operation depending on what the problem actually is... could be a burnt contact or a "soft" latching coil (ie. weak).

As for why it was running rough initially, potentially only enough fuel was finding its way to one side / one injector.

Anyways, hopefully swapping relays helped your problem. Of course, keep an eye on where that relay is now as it might lead to other "mysterious" problems down the road.

 

[Dennis94XLT]

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[MrShorty]

Where to get a fuel pressure gauge? -- Any self-respecting parts store should have one.

 

[Dennis94XLT]

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[budwich]

I haven't heard much about problems with regulators. My past experience with vehicles that have had over 300K Km on them is that fuel pumps are more a problem .... and off course their powering. Relays fail for a number of reasons from broken contact arms, to weaken springs, weaken coils (overheats from long term operation), bouncing (weaken contact arms), high resistance thru contacts (burnt from arcing) etc.... When you say "until this is solved", does this mean that you are still having problems even when swapping out the relay?

 

[custom20aj]

Sounds similar to what I had this last weekend (1992 Ex). The engine was getting WAY too much fuel. Black smoke and gas out of the exhaust. Shaking and sounded like it was running on 3 cylinders. It ended up that the diaphragm on the fuel pressure regulator had a hole in it. If I unplugged the vacuum line from it, gas would shoot out where the vacuum line attaches. This meant the gas was being "sucked" through the vaccum line right back into the engine, flooding it out.

One of the ways I found it was getting too much fuel was I would pull the fuse for the fuel pump, and it would clear up and idle fine after a little while. If I put the fuse back in, it would start chugging after 3 seconds.

The fuel pressure regulator is definitely worth a try. It wasn't cheap ($72.00), but is very easy to remove (two small bolts, a vacuum line, and the fuel line). It's located at the front passenger side of the fuel rail. It took 10 minutes to put the new one on. I would try that first, before dropping the tank to get to the fuel pump.

 

[Dennis94XLT]

Budwich: I haven't driven the truck since the last time it wouldn't start. I had just finished a real busy week and have a few days where I didn't have to go out so I didn't. I was hoping I would find the problem (by that I mean positively find it). Since I have erased that error code - and swapped the relay, all I can do is drive it and if it fails - read the codes to see if that code comes back if the problem occurs - then I'll be sure it really is an electrical problem to the pump. As it is, I could have a problem somewhere else and that error code may have been from me swapping relays a long time ago to see if that fixed the problem from a prior problem . Do you know how long an error stays active before it goes to the memory as a past event?. At the first reading with my error code reader, I got that code immediately, then after trying it several times, I got an all clear with that error retained in memory. I cleared the memory then got an all clear period.
I know swapping the relay could fix it, but I won't know for sure until it runs a few weeks without failing - and if it didn't, I would expect the A/C unit (where the relay went) to fail intermittantly.

 

[MrShorty]

KOEO codes aren't "stored" in any way. They are "hard faults" set during the KOEO test. CM codes are supposedly stored for ~80 drive cycles after the last time the computer noticed the fault.

 

[Dennis94XLT]

Thanks - that's interesting... That means the problem was occuring during the test and after I swapped relays, the problem stopped, then moved to the CM. Maybe there's hope for that being a bad relay afterall.
Do you know where I can get Bosch Relays? I tried a few stores nearby and they all had brands I'd never heard of (but they were cheap!).
Thanks

 

[93liftedXLT]

so how did you fix the problom? Great thread by the way