Is it possible for these items to malfunction and not receive a check engine light?

I'm having trouble starting my 99 explorer and I've read numerous post with the same problem. Some of them seem to be fixed by cleaning or replacing the TPS & IAC. I have no error codes.
This morning I actually got it to start but only by holding the throttle all the way open. Once it started, I could let off the gas but it ran terrible (very rough & shaky).

 

Yes it is possible. The PCM itself can be bad, or have a connector problem or otherwise not be getting power. In that case the system has a "get me home" mode where the injectors are set to a fixed timing and the engine can be run to get the truck to a repair shop. This can also happen with a fuel pressure problem or the temperature sensor not commanding a 'warm' engine also keeping the injectors in start/rich mode. In the case of the PCM being bad or not getting power, a code reader will usually come back with all 8s (8888) for a code reading which indicated that the computer is not communicating with the reader port.

 

Hey Michael,

Thanks for the reply. Is there any way to test the PCM? Because today I changed the IAC solenoid & the TPS and still the same problem, won't start unless I have the throttle all the way down. And even after it starts it struggles to stay idling. But I did notice it was running very rich. I tried to drive it a little ways to see if a code would show up, but it never happened. I think I'm ready to take it to the Ford Dealship to get it fixed. Do you have any other suggestions??

By the way what part of Oviedo do you live in? I'm in Alafaya Woods.

 

[updated:LAST EDITED ON 19-Nov-02 AT 11:08 PM (EST)]I have had a couple of TPS failures in Explorer and other ford EECIV systems that were not picked up by computer setting a fault code under driving conditions. I knew something was wrong by the way the vehicle bogged down when accelerating from a dead stop and hesitated in passing maneuvers. Failed TPS DID NOT affect start/idle in any manner along with slow & steady acceleration. IAC valve is only an up-down stepper and is not directly monitored by the PCM.

Generally, both will be picked up by the engine running diagnostic test series. IAC will be stepped up and down and engine RPM is monitored to determine if it operated properly and idle can be maintained in a set range of RPM's. It's primary purpose is to allow PCM to control idle speed and above that, it plays little or no role in overall engine performance.

Also, during the test, the operator revs and holds engine speed above 2K and a momentary wide open throttle shot is given. The TPS is monitored for changing signal during these operations.

Along with above, engine run test varies engine timing, fuel injector pulse width is varied and O2 sensor monitored for changed conditions, EGR (if equipped) is also stepped and monitored.

From what you described, I would suspect a problem similar to a flooding condition. Check the fuel pressure in fuel rails. 91-95 specs are 30-35 PSI engine running 40 PSI key on engine off. If higher than that, suspect fuel pressure regulator.

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