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Knotty Problem *SOLVED* / High Idle (Sometimes) - Blip Throttle It Dies 200 1993 200

posted by  RicoS on  Mon Nov 24 15:13 CST 2008
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last visit: Fri Oct 24 02:18 CST 2014
[RELATED]

The other day I saw a followup to a post explaining the resolution of the problem. It seemed like the logical thing to do, so, I thought I would do the same. For those who missed my original posting:


>>>The car has about 150K on it and it has been well maintained. The OBD codes were 121 (Fault in signal to/from AMM) and 123 (fault in signal to/from coolant temp sensor). No big deal, I thought. Before I went through the hassle of changing out the FI temp sensor, I first checked it from the ECU connector to make certain it was not a wiring problem. The temp sensor proved to be right on the nuts both hot and cold, so I left it in place.

At start-up the idle is often high (1500 RPM), sometimes normal but always whenever the throttle is blipped, the engine dies. Pressing and releasing the throttle normally does not cause the engine to die.

I then cleaned the mass airflow meter and its connector with no improvement, so I tried a known good AMM - NO change.

The fuel rail pressure is a little low both with and without vacuum (about 29 and 38), but I've seen worse on other cars. Pinching off the return line, pegs the pressure gauge. Manifold vacuum at idle is about 18 inHg. The AMM has good voltage to it and a solid ground. Sometimes the Check Engine light will come on (code 121, again), but it usually clears itself on restart.<<<


The fuel pressure regulator was faulty after all. I was starting to go esoteric which is the curse of any trouble shooting problem once it starts to defy resolution, when I noticed the fuel pressure became very hinky whenever I blipped the throttle. That coupled with the slightly below spec regulated pressures convinced me to change out the regulator. The new regulator held the pressures (with and without vacuum) at the same slightly low levels as the old regulator, but there was now no quivering when the throttle was blipped. The idle is now rock solid at 750 RPM with no stalling out when blipping the throttle.

Now, the next puzzle is why did a hinky FI fuel pressure regulator cause the OBD to return codes 121 and 123?

Rich





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