Hi All:

I need a little help diagnosing a very rough idle with my 1988 F-150 XLT Lariat 4x4. 351, Auto, EFI, with A/C and Power Steering. If it's important, the guy (clown?) that owned it before me removed the thermactor, it's plumbing, and and the cat, and the truck has now has dual muffers which replaced a set of cherry bombs.

Anyway... drove out to my buddy's place, truck was running perfectly. Came out, went to go home and tried to start the truck and it acted like it was out of gas, but turns out it wasn't. I got it started and the truck had a really ugly idle and still does. It seems to be missing, the engine is shaking a lot as it runs and there's a strong rich smell when it's running.

I've checked the secondary ignition and all seems in good shape. The cap and rotor look great, it has a set of champion truck plugs less than six months old, and I changed the IAC for a different idle issue which the replacment IAC corrected. I also changed the temperature sensor (the one for the computer, not the one for the gauge) at the same time as I changed the plugs. I'm not getting a CEL light, and I've checked the light when I turn the key. The cap, rotor (Wells), and wires (Niehoff) have about 25,000 miles on them, and the coil was replaced last year.

Any ideas? I'm thinking MAP sensor or O2 sensor, but I don't want to just throw parts at the truck, especially in this case as these are not cheap sensors.

 

Even if the CEL is not on it might be worth checking for any stored codes. Also pull the vacuum pipe off of the fuel pressure regulator and look for signs of gas in the pipe. If gas found replace the FPR. Check also for any cracked/broken vacuum pipes.

 

Those are good ideas... I think I'll try those this evening. I hadn't considered the FPR... that's a nice quick easy check. I'll have a look and reply back.

It is possible for a MAP to go bad without pulling a code?

 

Spend the last few hours doing some more troubleshooting

- Corrected a vacuum leak at the EGR valve. Didn't make a difference.
- Checked the fuel regulator for leaks, and pulled vacuum line from regulator looking for fuel leaing from the vacuum fitting. No leaks from the fitting and no external leaks
- Connected a vacuum gauge... reads between 15 and 18. Truck has 215,000 miles, so maybe a bit of a low reading is to be expected?
- Connected my scanner. No KOEO codes, but ran a KOER scan, and the scanner pulled code 42 - "EGO Indicates system always rich"

OK... with the black smoke from the tailpipes, I'll buy that. So I'm wondering which way to go next. Would anyone agree that if the scanner says the EGO sensor reads rich, and that if there's black smoke, that the EGO sensor is probobly working ok? I put a new temperature sensor in last summer, so probobly not that. I'm really thinking MAP sensor? Anyone have any thoughts?

 

Unplug your o2 sensor while the truck is running and see what happens. If its running rich that may be your culprit.

 

ok not to be harsh on Champion plugs... But... I have had several that got hot and the gap expanded and created a miss for me (thus i use motorcraft or bosch now)... Just a quick thought that even though the plugs are new you might want to go back and check your gaps...

 

I can pull the plugs and check for gap. Would a misfiring spark plug make it run very rich though?

 

There was a suggestion earlier that I try and disconnect the EGO sensor. I can do this, but won't this cause the computer to pull a code? Maybe complicate the diagnosis, or should I just give 'er?

 

i just suggested the spark plugs because that could cause the rough idle. and even if you disconnected the EGO and it created a code... that code would go away after reconnecting it.

 

Ok. Well I'll try disconnecting the EGO sensor and see what happens. I'll post back with the results. I also found a test for the MAP sensor as well, so I'm going to try that tonight too. Thanks everyone for the help so far.

 

Did a search here and found some old posts where guys were getting a code 42. Looks like lots of folks were going the same direction I was about to and it turned out to be a fuel pressure regulator.

Went to the store and got a fuel pressure tester. Attached it and found my fuel pressure to be over 100 PSI.... right past the last number on the fuel pressure gauge. Fuel pressure is supposed to be between 32 and 38 psi on these engines, so I changed the regulator. The truck is now purring like a kitten. I didn't think these things failed without leaking through the vacuum line, but I guess they most certainly can!

Anyway, if someone else is having this issue, hope this post helps someone out one day.

To everyone that responded earlier and tried to help, ALL responses were and are very much appreciated.

 

I have the same problem. Replaced about everything except sensors. Crawled around everything to check for leaks, but nothing. I will try the pressure regulator!!

 

No go. I can get it started with some difficulty and feather the gas to get it going, but will not hold RPM. If I keep the accelerator down, the RPM's will go up and down. Certain loads will cause a lot of knocking, and bucking...

 

bikeboy one thing it might be is the engine temp sensor. i just had the same symptoms you did.

 

Did you have the problem if the engine was "cold" or warm? Temp gauge seems to be working ok...

I'm thinking Throttle Position Sensor?

Here is what I replaced:
Cap
Rotor
Plugs
Wires
PVC
Fuel Pressure Regulator
Fuel Pump
Sucked out the fuel, replaced with new
Air Plenum Intake Gasket
Timing Chain / Water Pump

Checked for leaks in vacuum lines like crazy / sprayed starter fluid to find leaks, and nothing...

Engine will rev when I spray in the air intake...