QuestionQUESTION: Hello Gary,
I've been pursuing this issue for quite some time now. This idle problem occurred literally overnight. It was running fine. Then the next morning when I first started it, it would run up to about 1700 rpm then fall to about 300 rpm (nearly dying), then it would repeat - run up, fall down, run up, fall down. The cycle was about every three seconds. Now it is becoming very difficult to even start. I discovered during my investigation up to this point that it will idle smoothly with the MAP sensor disconnected. If I cap the vacuum port it will run at about 900 rpm, but as it warms up it will eventually stall. If I leave the vacuum port open and let it suck air it runs very smoothly around 1100 rpm with no hint of stalling. It will even take throttle and rev normally in neutral. However, if I attempt to put it in gear (automatic tranny) it will stall. If I try to reconnect the MAP sensor at any time it will immediately stall.
At this point the following parts are new in a effort to solve this idle issue.
O2 sensor
Throttle position sensor
IAC valve
MAP sensor
EGR Valve
Nearly all the vacuum hoses
Throttle body base gasket; Also cleaned and inspected the throttle body; All the vacuum ports are clear and the butterflies are snug and opperate smoothly.
I have also verified the continuity of all the sensor connectors and wires back to the ECU, and I have verified the bias voltages and grounds for the sensors that need them. I had a performance chip installed in the ECU years ago, so I have also put the factory chip back in the ECU to eliminate that possibility.
I am beginning to suspect that the ECU itself may be causing the problem. However, during my research I've read that the Coolant Temp Sensor can cause idle/running issues (this is one I have yet to replace), but most of the examples were after the engine had warmed up. Mine won't even run when it is cold. I haven't "tested" the coolant sensor yet, but when it is cold it measures 2.9K ohms across its terminals. It this a typical value? I may replace the coolant sensor just because it is relatively cheap, but I'm leaning toward a defective/failed ECU.
I'm not one for "shotgunning" car problems by throwing parts at it, but I sort ended up doing it anyway in this instance. Any input you have will be appreciated.
ANSWER: check fuel pressure
---------- FOLLOW-UP ----------
QUESTION: Gary,
Will weak fuel pressure really allow it to run normally if I keep it above an idle? Because it does take throttle and rev from 2000 rpm on up.
Still, how do I measure the fuel pressure? Where/how is the gage tied into the system? What psi should I expect to see?
Thanks.
Tony
AnswerYes it is possible to just have an idle problem it does not happen often but it does happen. There is no Fuel Pressure Test Port on these vehicles.
Turn engine OFF to relieve system pressure.
Disconnect negative battery terminal to avoid possible fuel discharge if an accidental attempt is made to start the engine.
Loosen fuel filler cap to relieve tank vapor pressure.
Uncouple fuel supply flexible hose in engine compartment.
Install fuel pressure gauge between steel line and flexible hose.
Tighten gauge in line to insure no leaks occur during testing.
Connect negative battery terminal and START engine and observe fuel pressure reading.
It should be 9-13 psi.
Turn engine OFF to relieve system pressure.
