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Sly
Premium Member
join:2004-02-20
Tennessee

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Sly

Premium Member

1994 5.0l F150 engine shudder problem

I have a '94 F150 that has an engine vibration at low RPM under load and at cold engine idle. It seems to smooth out a little once warm, but not much. The truck will shake under load in all gears at low rpm. It shudders when you first start it up when cold. Then it runs smooth once you get the RPMs up over 2000 or so.

I bought this truck used and it has about 140,000 miles on it and so in the process of trying to find the problem, I've done a bit of parts swapping. I initially suspected an engine misfire and so I've replaced most of the ageing engine components in the hopes that it would be one of them and for just good general maintenance.

So far I've replaced the coil, distributer, cap and rotor, ignition module, and plug wires. I performed an intake cleaning with throttle body cleaner though the intake while running and with seafoam through a vacuum line.

I've checked the engine out and have found no vacuum leaks. The engine timing is correct and there are no engine codes.

I haven't replaced the plugs yet but they are only about 2 years old and are platinum. I have a new set of motorcraft copper core plugs coming and will put them in. Maybe pulling the plugs will tell more about what could be going on. I haven't performed a compression test. The engine does not use oil or antifreeze. The mileage has dropped a lot since the shudder started a few weeks ago. The motor mounts look fine as far as I can tell.

If replacing the plugs does not work, then I suspect I may have a valve issue or a faulty fuel injector. But this is where the guessing begins. I don't have access to an engine analyzer and would like to avoid taking it to a shop if possible (I hate other people working on my truck). Any ideas on what to look for or how I might further troubleshoot this problem?

I don't mind parts swapping in that most of these needed replacing anyway. But I would like to get to the bottom of it soon as I don't want to keep pouring money into the engine without finding the problem.

As far as I know the timing chain has never been replaced. One other symptom I forgot to mention is that the tachometer will jump slightly around 2000 rpm if I accelerate quickly. If I accelerate lightly it will increase smoothly. When this whole thing started, the tach was jumping wildly on acceleration. That's why I suspected the ignition system and began replacing parts. Once I replaced the module and the coil, the tach smoothed out considerably but it still has that slight leap at 2000 rpms. Don't know if it's just an old tach or if the tach signal is dirty... Thanks.

shdesigns
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Stone Mountain, GA
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shdesigns

Premium Member

Check the plugs. My '88 Mustang starts running rough when the plugs get a bit dirty. I found the platinum plugs don't last, the electrodes on mine wer tiney and could not hold the heat.

If the plugs don't help, then injectors are probably next. See if there are any engine codes, they should be readable without a scanner (even my pre-OBD standard '88 5.0 can display codes on a meter or LED; most later will flash the check engine light but my '88 has none.)

At about 140k I did replace the injectors on mine. I'm not sure if by '94 they have enough smarts to tell if one is going bad. Did not make much of a difference.

You can try fuel injector cleaner. Add some seafoam to the gas, I also like the Lucas gas treatment.

One symptom of a bad injector is slow to start. A leaky injector can bleed off pressure and it take a while to come back up.

If adding cleaner helps but the problems return, you can try having the injectors cleaned at a shop. The basically pump a cleaner right into the fuel rail.

A set of injectors was about $200 back in the mid '90s. Now I see they are $120 or so on ebay.

5.0's tend to eat O2 sensors (have had two Ford techs say that). They start getting slow to respond. Has helped mine over the years.

Sly
Premium Member
join:2004-02-20
Tennessee

Sly

Premium Member

Thanks. It's not really hard to start. It just runs rough for a little while. Overall I am not too happy with the plugs I have. They are Bosch +4. I should have never put them in...

Maybe going back to Motorcraft copper will help.

mattmag

join:2000-04-09
NW Illinois

mattmag to Sly

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A agree with going back to an original Motorcraft copper plug. There are endless stories of decreased performance when plugs not specifically designed for a particular engine are tried.

Another potential issue with that engine is carbon build-up on the back of the intake valves. It acts as a bit of a sponge actually, and causes improper atomization of the fuel as it enters the cylinders. Valve de-carbonization is a job that should be performed by a shop properly equipped to do so, but first off I would install the proper plugs.

DeltaElite
We Dont Dial 911
join:2002-03-29
Tucker, GA

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check the fuel pressure too....I'm about to have to do the same with a Chevy Silverado.

ropeguru
Premium Member
join:2001-01-25
Mechanicsville, VA

ropeguru to shdesigns

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Another vote for plugs. At that age too, if it has not been replaced, and you are having idle issues when the AC kicks in, replace the IAC. I also have a 1994 5.0L F150 and had to do that a couple of years back. Got to the point the motor was sticking and cleanings did not help.

Sly
Premium Member
join:2004-02-20
Tennessee

Sly

Premium Member

Got the plugs today and should have them in tomorrow. This truck has also had a problem with idle speed at times ever since I got it. Sometimes when I push in the clutch to shift, the engine will rev up to about 1500 rpms and then start surging up and down a few times before it gradually falls to normal speed. I've suspected an IAC problem but haven't replaced it yet. Might just go ahead and order one of those as well.

ropeguru
Premium Member
join:2001-01-25
Mechanicsville, VA

ropeguru

Premium Member

Well worth it. Mine idled so much better.

Sly
Premium Member
join:2004-02-20
Tennessee

Sly

Premium Member

Well I replaced the plugs and the IAC and must say it idles MUCH better. It brought the RPMs up to normal and it quit surging when I push in the clutch. The old IAC was very carboned up. I attribute it to the previous owner using crappy conventional motor oil for 88,000 miles and never changing the PVC breather filter.

With that said, the idle quality is still bad. It still shudders when under load at low RPM. When I pulled the plugs I noticed that the plug in #3 cylinder looked fouled. Not sure if it's oil fouled or fouled because it's running too rich.

At cruising speeds and under WOT it runs excellent. I did an intake cleaning with 3M intake cleaner and I cleaned the throttle body really well and it runs MUCH better. The throttle is more responsive and it will break traction on the rear tires if I floor it in 1st gear. It didn't use to do that so I know it has more power.

My guess is that the injector is stuck open. It won't idle well, especially when cold and shakes under load, but under WOT and cruising conditions, it's perfect. I would suspect that if it were oil fouled and I had a worn valve guide then it wouldn't run as well...

I'll get a picture of the plugs uploaded soon and see what you all think. Like I said, I'm not sure if this is oil fouling or fouling due to excess fuel... It doesn't burn oil that I know of.

If I have a bad injector, what should I do? Should I replace all 8 of them or just the bad one? Should I go with stock Motorcraft injectors or should I put in something different? I think stock is a 20lb injector. Go with a 20lb or go up to something else? Of course, If I change the injector size I would be replacing all 8...
Tig
join:2006-06-29
Carrying Place, ON

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For the suspect injector, have you tried treating a couple tanks of fuel with Heavy dose of injector cleaner or seafoam? Just a low buck idea to try.
I'm not familiar with a 94 5.0l ignition, but when you mention distributor and problems under load I wonder about the vacuum and/or mechanical advance functioning properly.
As for replacing a failed Injector, I only replace the bad one and I stick with what the manufacturer spec'd out.

Sly
Premium Member
join:2004-02-20
Tennessee

Sly

Premium Member

I have a bottle of injector cleaner in it now. I'm going to run it for a few tanks in the hope that it will make a difference.

I need to do a compression test to verify cylinder health. Haven't done it yet...

shdesigns
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Stone Mountain, GA
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shdesigns to Tig

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Ford 5.0's have not had mechanical or vacuum advance since the mid '80s.

Yes, look at the plugs. If one is running rich, it should be browner than the others.

You can just replace the one injector. Personally, I would replace the whole set to make sure they are matched.

Sly
Premium Member
join:2004-02-20
Tennessee

Sly to mattmag

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I finally got around to working on the truck today. I took a stethoscope and listened to the injectors and sure enough, injector #3 was silent. The other injectors made a loud clicking sound but I heard nothing out of #3. So I have a bad injector.

Just for kicks I unplugged the spark plug wire on #3 to make sure that cylinder was missing and although it idled rough, it was no different than when it was plugged in.

I need to buy a new injector set. What do you recommend? Is going with Motorcraft really going to make a difference or could I do just as well with another brand that is cheaper?

Should I get flow balanced injectors or would that not really make a difference and waste money?

shdesigns
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shdesigns

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You can get rebuilt and flow tested injectors off ebay for about $100 a set. Have seen $200 new.

I replaced one of my Jeep injectors from NAPA. It was different brand and even looked a lot different. Still ran just as good as before (old injector leaked.) Cost $50 though.

If you were racing, then it might be worth a matched set.

Sly
Premium Member
join:2004-02-20
Tennessee

Sly

Premium Member

Thanks for the info.

I still want to do a compression test tomorrow if the weather is good to see if I have an engine problem. But at least I know I have a bad injector.