FedEx

Hey All,

1991 F150 - 5.0 L. 2WD. Red (and patchy bondo - going to paint soon)

Yesterday my truck suddenly started running really terribly. From how hard it shakes, it feels like it dropped a bunch of cylinders all at once. I had just dropped my wife off at work - truck ran fine there - when I started it up the truck was missing and engine was vibrating really bad - like nothing I've felt before. Barely kept idling. The vibration got much worse when I gave it gas. It's so bad that it almost feels like the engine is being driven by a sledghammer - and like it's only getting power through half of its rotation?

When I drove it home (less than a mile) it barely got up to 25MPH - had to floor it to even get it to 25.

Side note - when I was driving home, I floored it to try to get past the worst of the vibration: It did shift up, and the vibration smoothed a little (definitely still there though) and I noticed a high pitched, whining/whistling/blowing noise rising and falling over the course of 2-3 seconds as I was driving with it floored - the engine seemed to pick up and lose a little power along with the sound. Vacuum leak?? It still wouldn't get past 25mph. When idling in park, if I floor it, it will lug badly. If I vary the throttle, slowly increasing throttle as I'm tapping it, RPM's rise over 5-10 seconds - then top out - even under full throttle. Almost feels the same as a rev limiter - although it's not revving high enough for that - only 4000 rpm or so.

Anyway - here's what I have fixed:
*EGR tube - what a pain. Had to remove intake manifold, scrape gaskets, etc. Thought it had fixed problem, but later found out it had made almost zero difference - still runs rough.
*Replaced the EGR valve a few days ago.
*Replaced coil pack a month ago.

Spent a lot of time looking at past posts - nothing immediately jumped out as the problem.

I have a short list of things to check first:
I Have NOT looked at DPFE sensor yet - that is what I'm going to check/replace today.
I'm also going to buy a vacuum tester and check for leaks, etc. Would a vacuum leak cause such a bad problem?

Otherwise, I'm stuck. I've seen people posting on lots of possible causes: Oil system plugged, Cam timing off, various sensors. I'm going to start with the DPFE and Vacuum tests (cheap and obvious) but I would really appreciate any advice on what to check after that. Since this is our daily driver, I'm under the gun a bit. I'd really appreciate any help - I'm planning to paint this beast before the snow falls, but I can get back to that until it's running again.

Thanks for the advice -

Kevin

Fast Hampster

If it has a timing chain - it could slip. Check valves timing (might be done by running compression test).

FedEx

I replaced the timing chain and both gears two months ago. I REALLY hope that's not the problem - but... I suppose it could be.

farmer12

Nah I doubt that new of a timing chain would jump, Could just be a bad coolent temp sensor. But I suppose its possible your distributor spun a little bit. I dont think that would cause what your describing but its worth a check. And possibly your distributor cap cracked open. I've seen that before...cap cracks open and jacks everything up. I dont think a vacuum leak would cause anything like that, maybe a rough idle but not missing like you say it is. Keep us posted.

FedEx

Thanks Farmer, appreciate the info. I'm a decent wrench turner - but there's lots I don't know.

Quick update: I tried to swap out the DPFE sensor (on top of the EGR valve?), but they didn't have one at Kragen - so instead I just unplugged it - no change in the missing, etc.

I did, however, get a vacuum gauge. It read 12 in/hg. Pretty steady at idle, and at higher RPM. I know that's low - but I'm at about 4,500 feet so that should equate to about 16 in hg at sea level. Still about 5 inches low though...

Farmer, I did check the distributor rotation while I was out there today (Cap was fine). I rotated through 15-20 degrees - idle got worse at both ends, but was still bad at even the best spot. Good idea though - wish that was it.

I also pulled all of the spark plugs. Not sure what I was hoping to find - but didn't find anything really. No bad oil fouling - all looked pretty much the same - maybe a touch more oil on a few of them, but looked pretty normal. That being said, the truck's only been driven a mile since it started missing - not sure that's long enough to do anything to the plugs?

I'm off to Kragen for an inductive timing light - maybe I'm not getting spark on some cylinders.

Farmer - I'm wondering about the coolant temp sensor you mention? Would that possibly cause the missing/bad vibrations?

FYI, I watched the engine as I revved it today - at its worst, the top of the engine vibrates back and forth an entire inch- whole truck shakes as well. Makes me nervous.

I'd appreciate any other advice - if it's getting spark, I'm out of ideas to check. The weird thing is that it happened all at once? A friend says it's probably an ignition component.

Anyway, thanks for the advice so far. I'll definitely keep posting.

Kevin

Oh, by the way - while I had the vacuum gauge hooked up, I pulled every other vacuum device off the manifold "tree" and plugged the fitting. No change in the vacuum pressure. It was steady right at 12. Does that mean there are no (big) leaks in the vacuum system? Is 12 in. way too low - or is it okay? Thanks again all

farmer12

12 is a little low, but I think its adequate. I'd say 15 is optimum, for a average engine. I dont think its got anything to do with your vacuum or egr system at all. Its worth a shot on the temp sensor, I could be completely off my rocker on this but in my experience, that sensor can cause big problems. Its right next to the distributor on the left side, its got...umm I think, 3 wires going to it. Right above the thermostat housing. Its gotta be something in the ignition, I dunno if an igntion module on the distributor could cause a miss like that or not. I had one on my sis F150 I6 do kinda the same deal. Shut it down to wash the motor...went to start it and it was completely dead. Turned out to be the ignition module. But thats not something you want to just go out and replace, their not cheap. Anybody else have any ideas?

Good old Bill

Tried to answer the PM, I'm at my daughters house, 2nd computor and am having extreme difficulty, OK so much for my problems.

Have a question, 91 5.0, you mentioned a coil, pack but Farmer is talking about a distributor, what does this engine have?

Forget about the EGR, DPFE and even a vacuum leak at this time, the problems associated with them shouldn't have an effect on an engine running up in RPM. Am thinking an ingnition problem at this time. For a distributor, check rotor for spark tracks from center contact down side to distributor shaft, also check underside of cap for tracks anywhere and everywhere.

If this is a distributorless coil pack it could also be the problem. Don't have any old mans hints for you there, I have them on my 96 5.0 and 02 4.2, but no experience with malfunctions.

For this engine to run this poorly so quickly this is the area I would look at first.

Have another question or suggestion, its quick and simple. Pull spark plug wires or just disable ignition if possible. Crank engine with starter, does it sound even or is there a glitch in the sound of the starter. Am wanting to hear just the starter motor with a constant noise, no hick-ups from less compression because of a dead cylinder.

Wish I was there with you in person instead of this keyboard.

craiger01

This may sound patrinizing but have you replaced your fuel filter lately? I have had slight issues with a clogged filter. It almost sounds like you have a fuel/air mix issue. Also does this truck have acarb or is it fuel injected. If it is a carb you may have a bad accelerator pump, or if it is fuel injected maybe an injector or two died.

mitchalvin

I had a similar problem on my 93 it started off as a high load miss and gradually became a miss throughout the rpm range. Ended up being that one plug wire had worn enough to ground on the engine instead of firing the plug. Even if you have all the clip mine ran through the intake manifold in places making it hard to change the wires and places for them to rub.

FedEx

Reealllly appreciate all of these suggestions. Thanks very much - this gives me a great starting place.

After reviewing the posts I'm going to start with the least expensive/easiest and work up - hopefully we find it sooner than later.

Thanks, Bill, for dealing with the computer issues - your post is helpful. I'm going to pull the spark plug wires first thing and listen for compression - then I'll check the distributor cap for spark tracks. (last night at Kragen the guy suggested I switch out the rotor - so I did that. Didn't solve the problem, but I think it sounds a little stronger.) Answer to your question Bill: It has something I thought was called a Coil Pack, but Chilton calls it an Ignition Pack - but the truck also has a distributor with a SPOUT. I replaced Ignition Pack ($30 at NAPA) when the truck wouldn't start after it was hot - month ago - it solved that problem.

Farmer - I'm going to replace the temp sensor. Even if it is working - I'd rather know the one I have is good since it sounds like a critical part.

No worries about being patronizing Craiger - only bought the truck a few months back, and haven't replaced all the fuel filters yet. I'll replace the filters today. It's EFI. How would I diagnose a spotty injector? (keeping in mind half of them are buried under my intake manifold and I'm not sure that my Check Engine light works - it's never come on - and if you read my other post on this truck you'll know that ain't right )

MitchAlvin - I'll check all the wires. Thanks. They're all pretty accessible.

Excellent advice - great members on this board. I wouldn't know where to start otherwise. Thanks for the help all - Will let you know how it goes.

Fed