I recently scanned my car when the SES light came on, came up with a P0300 Random Multiple Misfire code.

The plugs were changed about 15-20,000 miles ago, and I dont have spark plug wires, and a bad coil wouldnt be likely to throw that sort of code.

So I turned my efforts to fuel pressure, as that would seem to make sense, plus it has the occasional hard start, and can be somewhat moody at high rpms at times.

I went down to my local Harbor Freight and snagged one of these http://www.harborfreight.com/cpi/cta...emnumber=92699 for about 9 bucks, I believe the girl behind the counter rang it up wrong, or it was on sale perhaps, cause there wasnt a price next to the item on the shelf. But anyway I digress.

I put it on the car, primed with engine off it never got above 40 psi, probably about 38 I guess. With the engine running it was about the same except the needle was jumping around madly between about 34-38psi I would say. I guess it would jump around like that because of the injectors constantly changing the fuel pressure?

When revved slightly it would just barely bump up above 40 psi, I didnt rev it much over about 2500-3000 because it was a dead cold engine. But it never topped 45 psi.

With that said, I looked in my Chiltons, and the listed fuel pressure it shows in there for my engine is 41-47 psi, which I would assume that to be at idle. One somewhat common problem for these engines is a faulty FPR, but I dont have the typical "gas in the little hose connected to the FPR after the engine is off" problem.

My check engine light did come back on, and I have noticed it missing from time to time, I havent gotten the code scanned yet, Im waiting on my own scanner to be shipped here.

Any thoughts?

Alright, requisitioned the help of my 9 yr old sister, and got some new numbers. Gauge was rock steady this time, I guess there was some air in there last time making it act funny.

On initial priming, with engine off, it jumps up to about 44 psi quickly and then goes back into the low-mid 30s.

With the engine running on cold start (higher idle) it was reading about 36-37psi and with the vacuum line removed from the FPR it would jump up to 44 psi... once the engine warmed up the pressure went down into the 35-36psi range and then would jump back up to 43-44psi with the vacuum line to the FPR removed.

Do you think this swing is enough to say I have a bad FPR? The only other symptoms I have besides the misfire code, is an occasional hard start, especially on a hot engine, and sometimes then engine will bog way down after you first start it and when you first start moving sometimes.

Heh, this may or may not be part of the problem, but my dad'* friend has a 2002 Olds Aurora 3.5, and he was experiencing the same symptoms you were having. His turned out to be a failing fuel injector. So it sounds to me like they need to be tested to determine that they are all ok.

I guess that'* a possibility, but Im having a hard time wrapping my brain around the idea of a bad injector causing a multiple misfire. How would you go about testing them anyway? Is it something I can do myself? because im really wanting to avoid taking this to a mechanic if at all possible...ill suck it up and deal with it before I do that, because its still running well enough to get me where I need to go.

Another update:

I did a "bleed down" test on it. I had all the connections good and tight, with pipe compound on them to make sure i didnt lose pressure that way.

After initial priming and then letting it sit for 5 minutes, I had lost about 7 psi, Custom said that his FSM said that you should lose no more than 5psi in such a test..

Soooooo do you think I need a new FPR? Or should I try something else?

7 PSI isn't that far off from 5 PSI. I don't think the FPR is to blame here? They should be cheap enough though just to swap out, aren't they?

Im starting to agree a little with it not being the FPR, because if its bad, I should be having gas in my vacuum line, and I dont, not a drop.

I wish they were cheap enough to just swap out, but the cheapest ive found is going to be 40 bucks or more by the time it gets to my hand...not cheap enough in my book.

I might try pulling the plugs, or at least the front ones, and see what they look like, and then Ive found several lists of things to do after getting a P0300 code, so I might start working my way down the list, especially the free things.