recent P0300 and now low fuel pressure

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/ctaf/Displayitem.taf?itemnumber=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.

Any thoughts?

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.

That price is about right for Harbor Freight. Autozone has fuel pressure gauges for around $20-$25.

I've seen the P0300 DTC set just running out of fuel. In your case that fuel pressure seems low. How old is the fuel filter?

Fuel pressure will vary due to engine vacuum and whether the vacuum line to the FPR is on or off. The GM FSM has better test procedures and specs to test fuel pressure and the fuel pressure regulator. When you take ignition to on, not start (fuel pump prime) you will see an intial spike in fuel pressure reading on the gauge and that should be within service manual specs. While engine is running fuel pressure will be lower when vacuum is connected to the FPR. Without the vacuum line connected the fuel pressure reading should be higher. If there is an excessive difference between comparative readings the FPR could be bad.

Fuel filter was replaced a couple months ago, not sure on the exact time period.

I will try some of the things you mentioned later on, perhaps when I have an extra set of hands around, its hard to look for a spike on the fuel gauge when you have to be inside the car to turn it on.


What would be an "excessive" difference between readings?

And does it seem normal that the needle would jump around like that? I mean it flat out moves to the point its blurry. It makes sense to me though in that the injectors would constantly be pulsing and making the pressure change.

I agree two people would help unless the hose on the gauge is long enough to bring it up to the windshield.

Excessive readings vary based on the year and model. Even same models may have different fuel pressure readings. For variation of the FPR it could be as low as 3 psi or up to 6 psi.

I do not recall having very erratic gauge readings but it may be normal for your gauge. Some oscillation is normal but no major down/up swings.

I went with the Actron model fuel pressure gauge kit because it has a bleeder valve to purge air out so you get a more accurate reading and minimize oscillation.

Here it is hooked up to my Alero.

http://i5.photobucket.com/albums/y186/lizzywiz/vacuum_3400.jpg

BTW - Disregard the readings. Alero fuel pump puts out a lot more psi than one for the 3.5L or SII 3800

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.

That is a big difference. I don't have the Intrigue FSM specs for the FPR on a 3.5L but for the SII 3800 it says anything over 6-7 psi swing or difference means the FPR should be replaced.

It would be consistent with the hard starting. Since the car is running and based on the specs provided I would say your fuel pump is good although to the low side but it may be lower due to the design of the Shortstar engine and the rating on the fuel injectors. If it were a SII 3800 I would replace the fuel pump because that is too low.

Good luck and let us know how it turns out.

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, a friend of mine said that his FSM for the 3.5 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?

Based on what you have told us and seeing the results versus what the FSM says I would say it is a good starting point. Something is not right and it would be better to resolve the issue.

Another suggestion is to get a better fuel pressure tester. I went to Harbor Freight and saw the same gauge kit on sale until Feb 27 for $7.99 plus tax. So I figured what the hell might as well get a spare. Well I'm taking it back and getting either an exchange or refund. The gauge sprung a leak. Also, I noticed there is a lot more oscillation in the readings than with the Actron. Plus the bad part is I noticed the readings between the two gauges were totally different. The HF one reading alot lower :confused: so I would not depend on the results it is giving. Just FYI.