91 Ford F150 trouble

This has the big V8 engine. Upon first starting it runs for maybe 10 seconds OK then starts shutting off and on just like the ignition is being turned on and off. The intervals between running and chopping off and on are not even.

Then after maybe 5 minutes (haven't timed this exactly) it settles down and runs like normal. Seems like something related to the engine warming up.

Any help appreciated on what to look for that might be causing this.

Thanks

Hello 55 50 Ron,

How about more info? Mileage, plugs mileage fuel, fuel filter to name a few,

Guido.

o2 sensors

I'm the 3rd owner so I'm not sure of mileage, it's either 93000 or 193000. It's just a "sideline" vehicle and doesn't get used a lot. Fuel filter has never been changed by me. Plugs have never been changed by me. I know I don't take care of this truck like I should.

Things I have repaired not related to this engine problem are: Wiper motor, turn signal actuator (under steering wheel center), a brake line broken when I backed into a pile of brush - oops, dome light switch in the left door pillar, starter,

But I have been careful with engine oil and oil filter changes.

I'm sure this doesn't help much, sorry.

Your 'big engine' appears to be the 5.8 and not the 7.3 of the earlier years.
Research shows the ign module bolted to the side of the dist.
Back in the mid 80's Ford used an inferior rubber on the ign pick up inside the dist. It was subject to breaking down and shorting out the wires for the ign pick up inside the dist.
That 'rubber' insulator was white in color.
Now if your ign pickup is not white in color where the wires come out of it and is NOT cracked and breaking apart, I would tend to suspect the ignition module instead.
Be sure to wipe off the base when installing and grease it with dielectric grease for proper heat distribution.
That ign module might be dist mounted and those were more subject to failure than the modules that were firewall located.
Parts book gives the dist mounted option subjecting the little parts to heat failure.
If mounted on the firewall, I would not really suspect it to be bad.
When it dies, Got spark?

No, O2 sensors are ignored during warmup.

trouble at only 30 years! what a piece of junk!

No - just 29 years! Have to keep it! I can't afford a $50,000 new one.

Have you checked to see if it shows any trouble codes?

Most likely you have a vacuum leak that the computer can still correct for when it gets into closed loop.

Start with the basics, fuel pressure and spark.

You can get a fuel pressure gauge from the loan a tool at Autozone. Have it on the port before starting, see if it drops when the engine tries to die.

I think that has a distributor? If so pull the coil wire partially off so you can see the spark. Watch for intermittent spark when it's trying to die.

That should get you some idea what direction to pursue.

You might be ahead to just have it towed to a shop and let them figure it out. The tow should be around $100, it does not take many unnecessary parts swaps to burn through $100 worth of parts and your labor.

I had a sudden "no start" problem with a '94 Chevy that I could not figure out. I sent it to a shop thinking they would replace the fuel pump. "Nope, the coolant temperature sensor is causing your fuel problem". It's been running just fine since then. I would never have found that.

I don't think a 91 gives scan data : (;

I would confirm grounds good at the battery and all leads that come off the battery ground are clean bright and tight both ends.

I would check for vacuum leaks most common where the plenum mounts to the intake.


I would run a KOEO test... Address any KOEO codes before I run a KOER test.... After warm KOEO codes repaired then run a KOER test.


Install Fuel pressure gauge and leave it on till its fixed...

Then hook up a break out box and check MAF, ECT, IAT, TPS, EGR sensor. The process is elimination to confirm sensors are good/bad are in range.

Those are a demon it takes some special equipment to diagnose them properly in a timely fashion. A good understanding of how the system works mainly because of no menu (scan-data)

I would not throw the parts cannon at it till I confirmed all of this... Its not uncommon for the fuel tank/tanks to be full of rust and the injectors restricted : (

On a 91 everything is suspect including the o2 sensors, ignition and basics... I have many stories where it was something simple it was a long road to get there... If you take it to a shop you will need a shop that cut their teeth on a ford with no scan data... A good question to ask do you have a ford 60 pin break out box and know how to use it...

There are no silver bullets until all basic have been confirmed : (

As HoboNC said, find a shop familiar with your era of Ford trucks. The shop I went to had the problem figured out in a few minutes.

Pre 1995 (or maybe 96?) used the OBD1 system.

You can still get trouble codes, but it involves either an OBD1 reader OR jumpering out some connectors to get the engine warning light to blink out the code.

how to retrieve codes
https://www.f150forum.com/f10/how-retrieve-trouble-codes-obdi-10907/

List of codes
https://www.troublecodes.net/ford/

Found these with a quick google search, you should do some reading to verify this will work on your truck.

As an aside...doesn't that truck have dual fuel tanks? Does it do the same thing on both tanks? I think (IIRC) they each have a fuel pump in them.

Good Luck

Fred

RusselAZ I think is onto it. For the first few minutes you are in open loop, when it goes to closed loop the computer begins to adjust the fuel mixture. Those years a common vacuum leak was at the plenum gasket. Also check that the EGR valve is not stuck partly open.

I would suspect either an ignition wiring issue or fuel restriction, should not run OK at all if vacuum leak when open loop.

Stupid me.
Naturally, after i went to bed last night I figured it out.
Runs rotten cold. Sometimes better than others.
Warm it might fail, chances are it won't.
Go out on a cold engine, fire it up, foot on brake and fire it up.
Place foot on gas and with foot on brake, push down on gas.
This "Power Braking" test is a fast way to load an engine and find ignition breakdown and usually is in the ignition wires.
Wanna see fun stuff? Pop the hood at night, fire it up and look for the errant spark shooting to ground.
Might be in the middle of the plug wires near ground or down around the plug boots.
Want more fun?
Alligator clip a wire from a long shank screwdriver and ground.
Work the screw driver around the plug boots and over the wires.
Your bad wire might show up real fast.

Your truck has the older EEC-IV system. This system does have built in diagnostics, but not to the level of the OBD-II systems.

When you first start it cold, it is in open loop operation. This gives it a rich mixture to start, then leans it out according to readings from the B/MAP sensor (or it may have a MAF sensor instead) and the IAT and ECT sensors. This is pre-calculated and programmed into the PCM. Since this is pre-calculated, things like vacuum leaks or bad sensor readings can upset the balance of the system. Ford began using MAF sensors in place of the B/MAP sensors some time in the early 1990s. Your system could have either one.

Contrary to what has been posted, the oxygen sensor on Fords has NO INFLUENCE on mixture unlike GM and Chrysler systems. Ford engineers consider oxygen sensors to be "after the fact" sensors. They are only there to monitor the overall functioning of the system. If the system is not working properly, the O2 sensor "sees" that and causes the PCM to set a code and light the MIL light (Check engine).

Your problem most likely is a vacuum leak given the symptoms, although a bad reading from a temperature sensor could make the PCM be fooled into going into closed loop before the engine is ready. Another possibility is if the throttle body has been cleaned, it raises heck with the PCM. Sometime in this era, Ford redesigned the throttle bodies to eliminate the need for periodic cleaning. Along with that they added a sticker indicating that the throttle body should not be cleaned. If your truck has that sticker, cleaning the throttle body can cause drivability problems, but if it does not have that sticker, it may need cleaning. 1991 was a transition year. Mater of fact, all through the 1990s there were a lot of transitions. From OBD1 to OBD2, from R-12 refrigerant to R-134a, from B/MAP sensors to MAF sensors, and the list goes on.

There really isn't a short answer to your question. You need to do some diagnosis before you take it on a ride through the parts department.

Thanks for all the responses. I'm not one to "throw parts at it" so the diagnostics path will be followed either by me or my favorite mechanic's shop.

Thanks again and I'll post back to tell what the problem was.

Vacuum leaks can cause cold start issues and then might cause a higher than normal idle after warm. I'm sure you probably followed all the vac lines and sprayed something to look for leaks. The PCV valve and its hose are almost hidden at the back of the upper intake on passenger side and to pass valve cover, easy to miss visually or with spray.

The component that is responsible for idle speed is called the idle air control valve ( IAC ). These were notorious for failures, and sometimes can be removed and cleaned out. The IAC is mounted on the inboard side of throttle body beside where the two intake tubes mount. If a vac leak is not found it's possible the IAC could be gummed up or simply failed.

As mentioned temp sensors (coolant and air ) can cause problems like this too. The coolant temp sensor ECT in on the lower intake manifold right under the IAC. The air charge temp sensor ACT is on the lower intake too, about center line on the drivers side where the upper intake mounts to lower (2 wire sensor) These sensors are not interchangeable but look almost identical.