Question1. I bought a well-maintained '90 Montero (3.0L MPI V6, auto) with 114,430 miles in April '03. Records show that the IAC had been replaced in '99, and again, along with the ECU, just a month before I bought it. I had only owned the truck for about a month when they both failed again. I had them replaced at the dealer. I should mention here that before and after replacement, the IAC has never seemed to function. Problems surfaced again seven months later. Back at the dealer, they claimed that the IAC had been damaged by a coolant leak caused by a faulty o-ring in the fast idle air valve assembly. The warranty provided no coverage in this case, unless I was willing to authorize the replacement of the "unserviceable" air valve assembly ($770+). After some haggling, they let me keep the new IAC (no warranty), and I towed my truck back home. I rebuilt the "unserviceable" air valve assembly, replacing the faulty o-ring (50 cents, Ace Hardware). And as before, the new IAC does not work either. I've had to set the BISS and TPS to compensate. A diagnostic shop recently confirmed that the IAC was getting a signal, but was not responding. They would not confirm if they'd tested the IAC itself. I've inspected all related components, and have performed all tests outlined in Chilton's and Haynes. IAC responds to the 6v hotwire test.
2. This next problem is the most recent and important, but unrelated to that above. After driving somewhere and parking the truck, it will stall while driving within a mile of restarting. If I let it sit, I may be able to start it, but only if I put the throttle to the floor. The longer it sits, the easier it starts. I've just replaced the fuel pump and filter, which has brought no changes; pattern repeated itself. In the past 6 months, I have disassembled and cleaned the throttle body, cleaned the PCV valve, replaced the O2 sensor; battery and plugs are also good. I would appreciate any help you can provide in solving these two problems, and I think that what you've done to help others is outstanding. Thanks, Anthony.
Respectfully,
TMD
AnswerToby,
Your fisrt problem is a tricky one. Most all mechanics know what the IAC motor is and what its function is, but few understand why they can fail. It's a remote possibility that the new IAC motors and/or ECU you got were bad. Most of those are not new, but rebuilt. If you can hotwire the IAC and it works, than there still may be a problem with your ECU receiving the signal back from the IAC, thus not regulating fuel flow. You may want to pull your ECU just to give it a visual inspection for any leaking capacitors or damage.
Your second problem may actually be related to your first problem. Possible causes of this situation include malfunctioning fuel pressure regulator, slow responding Idle Air Control Valve, and to a lesser extent a faulty gas cap.
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Fuel Pressure Regulator (FPR):
A cylindrical vacuum operated valve that has a small vacuum hose coming out of the front side and a larger fuel hose coming out of the back side.
Function:
Maintains proper fuel pressure in fuel rail based on vacuum signal from intake manifold. Excess fuel is routed by the FPR back to the fuel pump.
Test Procedure:
While engine is running (exercise extreme caution when performing this test to prevent explosion and fire) After hot engine has been setting for approximately 10 minutes, remove the FPR vacuum hose where it attaches to the intake manifold. If gasoline is present in this hose, the FPR is defective. Remove fuel return line from the rear of the fuel rail. Attach one end of a long length of hose to the end of the return fuel hose coming off the end of the fuel rail; place the other end of the hose in a suitable container away from the car. Start the engine. Fuel should immediately come out of the hose in substantial quantity. If only a little fuel is coming out, or in sporadic spurts, the FPR is defective.
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Slow Responding Idle Air Control Valve\CIS Motor
Function:
At idle, the IAC valve opens and allows the necessary amount of air past the closed butterfly valve in the Throttle Body so the engine will idle properly. Note: It is possible that the IAC is responding slow due to a faulty signal from the ECU. Sounds like you've tested the IAC thoroughly, so I won't go over that.
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Gas Cap
Test Procedure:
After hot engine has been turned off for several minutes slowly remove cap. If a hissing or rushing air sound is heard then the vent mechanism in the cap is defective. Try a new gas cap.
Good luck!
