diagnose this

I had the same set of symptoms on a '99 Explorer.

It took almost 2 years, and $1,400, of replacing/repairing one thing another until the "upper and lower intake manifold gaskets" were replaced. The mechanic tried that after allowing butane or propane to be released in various places as the cold engine was idling, and the fact that the engine speeded up when the butane/propane was released in a certain place suggested to him that there was a leak there, and that the leak was allowing the butane/propane to be sucked in and burned with the gasoline. His theory was that this leak was only there when the engine was cold, and that when it warmed up the metal expansion closed the leak; when the engine was cold, and the leak was there, additional air was being sucked in and diluting the gasoline/air mixture so much that the missing and stumbling was being caused.

(What I'm reporting here is my experience, and what my mechanic told me. I have no automotive skills, and thus offer this only for consideration.)

Best regards to all.

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This is the standard procedure for finding vacuum leaks of all sorts. And if you encounter rough running problems that seem random, one of the first things to do is to start looking for vacuum leaks.

It is an excellent offer, though.

--scott

Just curious, but did you have intake manifold gasket leaks and NO coolant loss or white smoke at startup? I'm having trouble imagining a manifold gasket leak without coolant going somewhere, either inside the engine or outside. Unless it was ONLY the upper gasket that was leaking. Seems kinda unlikely.

Yea, well, if it wasn't for Hayne's and this newsgroup my Explorers would have been in the junk yard many years ago.

There's your problem--you are going faster than what the speedometer goes up to.

Vacuum leaks when cold a fairly common with most flavours of the 4.0 Explorer engine that feature the plastic upper manifold... Easiest way to check for the concern is to smoke test the intake after a cold soak... this type of concern generally doesn't have a coolant leak to accompany it.

The upper intake is sealed to the lower intake with six individual "o-rings" ... any or all of these can leak.

Generic answer: 2 and 5 maybe seeing an improper mix, or may be being 'scrubbed' from a coolant leak into the cylinders.

Yes, one possible guess might be a small coolant leak that only leaks when the engine is turned off. That can foul a plug for a little while at start up. But this thread is old enough, by now one would expect the missing coolant would be noticed.

-jim

"Ulysses" wrote

No coolant loss noticed, no white smoke at startup. I'm not defending the diagnosis or what was done, but this last fix seems to be working, with a couple of thousand miles on the car since the repair. Over the previous 2 years and 10s of thousands of miles the problem always resurfaced (after each of the several other attempts to fix it) the first time I started the car with temps below 50.

Previous attempts to fix the problem included replacing plugs, replacing wiring to the plugs, replacing an/the oxygen sensor, replacing the fuel pump relay, and a couple of other things I don't remember.

Best regards to all.

Bad engine temp sensor?

There doesn't seem to be any missing coolant or any smoke out the back.

It's keeping a steady 35 psi.

Thanks. I'll keep this in mind if my '97 starts doing what yours was doing. As for the other fixes most of them sound like things that probably need to be done anyway so it doesn't sound like you wasted much money on them.

Problem solved.

Shop didn't like the Motorcraft double platinum plugs that were in it, said it shouldn't be using platinum plugs. Told them the plugs only had 8K on them. They couldn't find anything specific with their "test equipment" so they recommended an injector cleaning. Told them to go ahead. Very slight improvement but still not running right. Shop said they were 98% sure new non-plat plugs would fix it. Told them to go ahead. It fixed it. Now it runs fine. Very strange. I've used motorcraft plat plugs for 137,000 miles with no problems till now.

Check the underside of the coil pack to see if the potting compound is cracked. It takes less energy to fire standard plugs than plats.

some engines just don't like platinums, especially older engines that may have a little blowby. They tend to foul and never burn clean. I hear of this most often with Bosch FWIW.

nate

I've heard that about the bosch. But I looked at these plugs (Motorcraft) before I took the truck in and they were clean, the "center" plugs were "too clean" you might say, the porcelain on them was as white as a new plug. The only thing I can think is that those cylinders were running a little lean and the plugs were getting a little too hot and the deposits on them turned to a white glaze that was shorting out the spark. Probably new platinum plugs would have fixed it too. However, you'd think that would have shown up on their test equipment and they claim they didn't see anything that looked bad with the plugs or wires.

When I see Motorcraft I think original. Were these the original plugs from the factory? The original plugs should be opposite plats. Two different part numbers.

They are whatever is sold retail, maybe it's Autolite or something, whatever the "ford" brand is. They are double plats, you can't get the original single special side for the left and right that they put in at the factory.