I have a ?98 Ford Explorer that has stumped my excellent local mechanic and the Ford dealer. I am the original owner and have driven it 145,000 miles. It has a start-up problem that will go months without showing up and then will happen every day for weeks. This problem has persisted for about a year. It always seems to start up fine initially, revving up to 1500 rpm. In failure mode the idle will quickly fall to 500 rpm and become very rough and the engine shakes like a dog defecating peach seeds. Usually it will stall in a few seconds or I kill it and start it again. After up to a dozen restarts, the idle will drop to 500 rpm but then oscillate back up to 1500 rpm and repeat this cycle with a period of about 6 seconds. That?s when I know it will make it. The low idle point gets higher and higher until ultimately the oscillations converge on 1000 rpm and the car is good to drive until the next time I stop the engine. The engine never stalls once it is running and is good for long distance freeway driving and stop-and-go traffic.
My mechanic has replaced the fuel filter, cleaned the EGR valve, and replaced the temperature sensor. The Ford dealer has never been able to duplicate the problem in spite of holding it for 2 weeks, even if it stalled starting right before I took it in. I have run several tanks of gas with fuel injector cleaner.
Any ideas would be appreciated, or if you could get the cash for clunkers deal revived I would jump at the chance to replace it. As a Caltech engineer, I realize the MIT-Caltech rivalry might cause you east coasters to pass on this problem, but think of the prestige MIT would gain in California if you could solve this problem.
Thanks,
Roger from San Luis Obispo
Injectors?
“shakes like a dog defecating peach seeds.”
-blink- Wow. 
We usually say, “like a dog trying to defecate razor blades.”
But seriously, the problem might be with the Idle Air Control valve. This valve is what controls the engine idle speed under all conditions. Sometimes these IAC valves become defective where they hunt for the proper position for the idle condition. This then causes the idle to be too low or the idle fluctuates up and down or both.
A quick way to check for a defective IAC valve is locate the valve, start the engine, and with the handle of a screwdriver rap on the valve. If doing this effects the engine idle or the engine stalls, the IAC valve is defective.
Tester
Thanks for the suggestion on the idle air control valve. I forgot to mention that the IAC was replaced as one of the attempts to solve the problem. I will try the test you suggested anyway.
If I had to make a guess on a problem like this with no diagnostic info, I would shoot from the hip and say reseal upper intake manifold. These plastic upper intakes are notorious for leaking cold and sealing when warm. Is your check engine light on? Do you have a P0171, P0174?
Thanks for the suggestion - it sounds worth a try. The check engine light does not come on. No error codes have been read as the vehicle never failed for the dealer.
“Mission Control, Cal-tech here, we are experiencing the dog shooting peach seeds syndrome, we ask you hold the planned download until the problem clears”.
I can comment that it appears your vehicle has not suffered from a “shotgun” approach in parts replacement, at least the cost inncured has not been great. Fuel filter,EGR cleaning,temp sensor, all good ideas. Doing nothing when you cannot duplicate also a good idea. How about an event data recorder? this is sort of a wish list type request as who can keep on of these hooked up when the problem is so intermittent? Have you looked for TSB’s? I will and I better not find any.
EDIT: You really need to take a look at TSB’s for your SUV. I have found 3 realy good possibilities no matter if your engine is the 4.0 or 5.0. I am very disappointed that an engineer would not explore the possibility that the answer to his concern has already been diagnoised and a fix explained in a TSB.
Good idea. My mechanic, who has babied my 4.0 Explorer since new, did check the TSBs, but he is only close to being infallible. I will look at the TSBs and ask how he resolved any that look applicable.
I doubt that any of the TSBs is associated with a leaking intake manifold because my mechanic would have been all over that, and that is the most promising suggestion so far because of the temperature dependence.