So the girlfriends MSM is running like donkey *****. It's throwing Lean off idle codes and is very hesitant in power delivery. LTFT is 20% indicating a pretty sizable lean condition. I did the usual vacuum leak checks. Even did the old spray starting fluid around couplers and seams to see if the idle rose any. No dice. So I went ahead and hooked my fuel pressure gauge up. I get a reading of about 35psi at idle and it will dip to 30 under load. This seems really low to me. What is the spec for Fuel pressure? Most modern returnless systems run 60psi or better. The fuel pump is a couple months old and the car has always seem to hard start when it sat for a while. My guess is the FPR.
Your thoughts?

 

It should be 65psi at all times. Hope it doesn't kill the car completely.

 

If you have new fuel pump and it is getting good voltage, I would get the injectors cleaned next. I have never heard of a failed FPR on a Miata. They tend to be a very reliable part. I have seen a few relays go bad too. Usually a new fuel pump solves this problem though.
Stephanie

 

I had a 1.8 FPR fail on me a while ago. Luckily it happened while I was tuning/staring at the wideband.

I haven't played with returnless systems enough to tell you where to go from here, though. You want to try and pinch the return line off - if the pressure spikes, the FPR is your culprit.

 

I would suspect the new fuel pump. What was the reason for replacing the fuel pump in the first place?

 

Agree with Dustin

 

FPR. For ***** and giggles, disconnect your TPS, drive around some and let me know how the car feels then