1992 F-150 idle problem

I tried to help my brother out with his truck, 302 EFI 2wd. It runs great going down the road, acceleration is fast and smooth. When you start it up, either warm or cold, it idles rough, and slow for anywhere from 2 to 5 minutes. It used to race up and down, but that has gone away, don't know how.(warmer weather?) It only had 2 codes show up when scanned, EGR volume low, and fuel pressure low. The egr problem was the sensor above the egr valve now fixed. I will be checking actual fuel pressure this weekend, but that doesn't seem like it would cause an idle problem, as it runs all the way up to each shift point at WOT without any missing. I replaced EVERY vacuum hose, and sprayed with carb cleaner around every intake connection I could find, with no leaks found. Any ideas?

Reply to
Steve Walker
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Warmer weather will reduce the severity of such a problem -- exactly!

This did not seem right to me so I took a quick look at all possible engine codes for a '92 model 5.0 liter Ford PU. I didn't find a "low fuel pressure code." There is nothing on that engine that directly monitors fuel pressure. There is a code for "oxygen sensor indicates system lean." Low fuel pressure IS ONE POSSIBLE CAUSE OF MANY for this code. This code can easily be caused by a vacuum leak. Vacuum leaks cause exactly the bad and even possibly "hunting" idle that you describe. At idle the rogue air coming through a vacuum leak is a high percentage of the total air intake and greatly disturbs the mixture. At wide open throttle it is a drop in the ocean and so -- despite common false belief -- the vaccum leak has little or no effect.

Correct -- probably not a fuel pressure problem if the truck winds out with vigor.

Good idea!

This truck probably has a vacuum leak at the intake manifold that closes with warm up. VERY typical. We have found this situation hard to diagnose until we got our smoke machine which makes it child's play to find intake manifold leaks on a cold engine. We hook it up to an engine that has sat overnight and run the test without ever starting the vehicle.

Less likely is a spark plug that was dropped during installation or not checked and has an almost closed gap. It will light the fire at high rpm but not at idle. The dilute fuel mix at idle requires a wide gap whereas wide open throttle much less so. One reason this is less likely is that if this is the cause the problem would be expected to persist when the engine warms up.

Don

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Reply to
Don

First, as Don indicated, there is no fuel pressure sensor on that vehicle - therefore, no code possible. That vintage did have problems with fuel pressure regulator failure which can and does cause idle and low fuel mileage problems. If you have fuel in the vacuum hose to it, don't bother to check the pressure - just replace it. They are not adjustable unless you buy a specialty performance unit. Your most likely idle problem is the IAC and a filthy throttle body. I know it has a tag on the side saying not to clean it but, there comes a time that it has to be cleaned. If you remove the coating, it only means it may have to be cleaned again sooner than normal. Use a good throttle body cleaner. Ford has a good one as well as some in the aftermarket. It is also possible to remove the IAC from the side of the throttle body and clean it making it return to satisfactory operation. It may require removing the motor part and soaking the valve portion in a good solvent overnite. If that doesn't do it, you will need to replace it. The IAC will not usually set a fault code if it is only dirty - just cause low/erratic idle. BTW, just spraying carb cleaner into it will not do the job. You need to just remove it. The throttle body can be cleaned pretty well with a couple of old tooth brushes while holding the throttle wide open - do this with the engine off!

Good luck Lugnut

Reply to
lugnut

A stuck IAC will cause a low idle but not rough or hunting. Also, it sounds like the code other than the one for EGR valve position is likely a "lean mixture indicated" -- perhaps the OP can give us the fault code number. You certainly can't go wrong cleaning a Ford IAC or throttle body, but my money's on a vacuum leak.

Don

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Reply to
Don

-- perhaps the OP can

code 334 egr valve position sensor signal out of closed limits

code 556 low fuel pressure

I didn't scan this, my brother had Autozone do it, and this is what they told him.

I will clean the throttle body & IAC this weekend, if he hasn't taken it to the local Ford dealer for a better examination by then.

Reply to
Steve Walker

I had leaned toward the IAC because he indicated rough and slow idle. The slow part doesn't seem to be typical of a vacuum leak unless it is really a lulu. He had also mentioned it no longer has the high idle feature on cold startup which to me would also suggest IAC stuck. A bad FPR allowing excess fuel pressure on these will act like a stuck choke and the exhaust will burn your eyes if you stand near the exhaust. This will also cause it to idle slow if it is bad enough to cause flooding as I have seen on a couple. It would be nice to have the codes if any. Usually, you will get a lean limit reached code if the FPR is bad. This is not the same as a lean mixture code which may result from a vacuum leak. Sorry, I do not have the different codes on my desk to look at. I have seen some reach for the EGO sensor with the lean limit code instead of checking fuel pressure first. They knida shoot the messenger.

Regards

Lugnut

Reply to
lugnut

335 is the EVP sensor

556 (according to my charts) is fuel pump relay primary circuit fault - Power / Fuel pump circuits It is either engine running or in memory. You may need some help to track this one if it is running. About the best you can do is check circuit voltages looking for a bad connection. I suppose the pump relay could give you this. It is in the box on the left fender.

Regards

Lugnut

Reply to
lugnut

Code 556 is no such thing! It exactly what Lugnut says -- fault in primary fuel pump circuit. Given the engine runs I would not be concerned about it at this time. The Autozone scan was worth no more than your brother paid for it.,

Pleease let us know.

Don

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Reply to
Don

Actually, in colder weather, at startup the idle would race at about

1500-2000, go down to normal, then pick back up to 1500-2000 and repeat over and over for about a minute or two.

A bad FPR

I forgot to mention we replaced the O2 sensor this spring, as he caught some debris on the road and ripped the wires up.

Reply to
Steve Walker

Sounds like the IAC is working and trying its damnedest to deal with a vacuum leak.

Don

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Reply to
Don

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