Running rough and stalling - error 543

[pmeade]

I have a '94 LTD 4WD with 120k miles. It will start up and idles fine for about two minutes, then the idle gets rough. The CEL comes on regularly.

If I put it in drive or reverse it will usually stall out. When it stalls, I can immediately start it again. I can keep it going by giving it more gas, and can even get it up to higway speed. However, there is definitely a lot less power than usual.

I pulled the following error codes:

Key On Engine Off:
543 - Fuel Pump circuit open connection - Electronic Control Assembly to battery

Key On Engine Running:
172 - HEGO sensor - voltage indicates "lean" (Bank #1)

543 - Fuel Pump circuit open connection - Electronic Control Assembly to battery

Fuel pressure is between 32 and 38 PSI as tested near the FPR.

I've cleaned the MAF, put in a new fuel filter, and a new fuel pump relay. I checked my plugs and they all look fine (light gray)and my wires are about 1 year old. About six months ago I changed the O2 sensor on the passenger side because I was showing a 172 error - it never went away.

I have seen references on the internet where they say 543 is usually the fuel pump relay - I put in a brand new one, and have ruled it out also by jumpering pins 30 and 87. The fuel pump comes on and stays on.

I checked the vacuum hose from the fuel pressure regulator and it was dry, so I think the FPR is ok.

I think the next step would be to replace the fuel pump, but that is a lot of effort and I'd really hate to do it and find out it isn't the cause.

Any ideas??? I'm scratching my head with this one and would really appreciate any tips you can throw my way.

 

[MrShorty]

The fuel pump appears to be running just fine in spite of the 543. Put a voltmeter in at the inertia switch and make certain that the voltage to the fuel pump is steady and not erratic. If the voltage (while the pump is running) is steady, then I expect the cause of this code is in the fuel pump monitor circuit (between the PCM and the point where it is spliced into the main fuel pump circuit I think at the relay). Where your fuel pressure is good and the fuel pump is running, I don't think this is likely the cause of your driveability issue.

I expect your driveability issue is related to the o2 sensor code. You don't list separate memory (CM) codes, and my code lists tell me that a 543 can't be a KOER code, so I'm wondering if your "KOER" codes are actually "CM" codes (see my "notes on pulling EEC-IV codes" in the EEC-IV forum). If you haven't yet done it, then warm the engine up and run the KOER test, then delve into diagnosing the O2 sensor code. I would probably start by checking the wiring to the O2 sensor, then switch the two O2 sensors to see if the code follows the sensor.

 

[pmeade]

Thanks Mr. Shorty!

I'll try the tests you suggested and will post my results for others to see.

I definitely received the 543 while doing the KOER test. It was also present during the KOEO test, so I don't know if it was persistent or not. I'll read your section on testing and re-test.

Good suggestion on swapping the O2 sensors. I bought one and can put it on, but that is $55 that I can put towards whatever else it might be if I use your option and find that it doesn't follow the sensor.

I wasn't able to see any obvious leaks in the vacuum hoses, but I was told by a friend I should do a leak test using propane. Do you think this is necessary?

Perry

 

[pmeade]

Woo hoo! I got it fixed.

On the advice of Mark Fosnaught at Clintonvilleauto.com I checked the EGR valve. It was only $18 so I decided to replace it and viola - problem gone! The truck is working just like it should.

Thanks Mr. Shorty for the advice. It kept me hoping that I would be able to fix this myself!

Perry