Bad Fuel Pressure Regulator or Fuel pump?

misfire cylinder 1. check plug wire and dis cap and injector and harness on cyl 1.

might do a compression test too on all 4.

any change in running if FPR disconnect? my reference says 40-55 PSI for the pump.

cylinder 1 misfire. have you tried a plug wire change on cylinder 1? have you inspected cap for defect/wear?

To clarify, after changing plugs and adjusting valves it only produces code 1399.

I unplugged each injector one at a time while leaving the other 3 intact and all resulted in a noticebly reduced rpm. I believe that rules out a bad injector for Cyl1.

When disconnecting the FPR and plugging the vacuum line the rpms rise a bit and rpms are buttery smooth.

I am fairly confident that the issue is fuel pressure related...my new question is: in your experiences does the low fuel pressure, but one that rises when FPR disconnected indicate a weak fuel pressure regulator or a weak fuel pump or both?

I bought the car for my daughter when it had 165K on it from the original owner, I'm not certain if the FPR and/or Fuel pump are original. At least the FPR appears to be OEM.

Thanks again.

I would suspect a bad fuel pump or a leak somewhere in the supply line. The intank filter can leak therefore the fuel pressure would drop.

Attach a fuel pressure gauge and with engine stopped, turn on ignition for a few seconds. Turn Off and On again. Repeat process a few time till you get the maximum fuel pressure available and record it down.

Clamp the return hose from FRP and repeat the above process. Note if there are any significant changes.

For both steps, after priming up the fuel pressure, if the fuel pressure are within specs, turn off ignition and monitor the fuel pressure. If it drops too fast, you have a leak and I would suggest checking the intank filter.

Fuel pump replaced today. Same issue: P1399. I drove the car myself and idle hunts all over the place (going to test the IAC, but no IAC code) and there is a pretty bad stumble just off idle and then rough running until about 2000rpms then "seems" fine.

Did you replace/test the intank filter?

P1399 is a pending misfire code.

A bador dirty IAC would result in poor idling, surging and stalling.

Replaced the intank filter when I did the pump. IACV replaced, same issue. MAP sensor removed; vacuum port in TB plugged and mitivac on MAP shows that it is functioning properly. Swapping known good ECM later this week. Cooling system properly bled. If the other ECM doesn't fix the problem I am at my wits' end.

You'd think with 542 sensors on the car it could give a more specific code than random misfire.

random could mean too lean or too rich like a vacuum leak might cause or leaking regulator. but yah, car computers are a joke in some ways. probably because of the non proprietary mandate. unique knowledge is POWER/MONEY.

Bleeding of the cooling system is not all. Did you check the IAC coolant hoses for clogging?

Yes, when I replaced the IACV.

Perhaps this will help narrow down the issue:

1) Only happens when warm

2) When I replaced the IACV I left the old one plugged in and zip tied to the fire wall. I watched the valve and when the motor idle was hunting the IACV wasn't doing anything....nor was the new IACV since it wasn't plugged into the ECM. Logically, this hunting shouldn't be IACV related.

3) The idle drop and rise seems to be "timed" and an even amount....like about 500 rpms at a time every 7 seconds until it almost stalls and then goes back up to 1200 ish, then 700 then almost stalling, repeat.

Hunting might not be caused by the IAC. The PCM tries to get the correct idling based on information received from the various sensors. and keeps enriching and leaning out the fuel mixture resulting in the hunting.

With the new IAC instaklled and not plugged in, the engine would definitely hunt as the IAV is preset to idle at around 1400 rpm and this idling speed is too high for the PCM so it tries to control it causing the hunting.

Understood, and not a single sensor is throwing a code. Maybe I am incorrect in assuming that lacking any other code that the ECM must be happy with the signals being received from all the other sensors.

The IACV plugged into the computer isn't doing anything corresponding to the rpms and bench testing the suspect IACV shows everything is functioning properly.

This other ECM couldn't get here soon enough...

does seem strange the IAC did not move when it was off.