Difficult P0300, looking for more ideas

[zuper8]

Been dealing with this issue for a while, my 96 SSEi idles fine but stumbles and makes a fluttering sound upon acceleration. P0300 comes and goes, has come on flashing occasionally when accelerating hard like merging on the highway.

Tested spark at the wires with an HEI spark tester, have spark on all 6. Plugs are new. ICM and coils have been replaced with used ones that were working when removed from my old 96 SSEi.

Went over numerous times with brake cleaner and smoke checking for vacuum leaks.

Repaired cracked EGR tube with JB weld.

Replaced FPR.

Replaced fuel filter and repaired rotted fuel/ evap lines.

Suspected that the tube for the IAT in my FWI is breaking away from the rest of the coupler, gooped some black RTV around it for now. Might not be leaking but just wanted to be sure.

I already did the intake gaskets earlier this year, torqued the intake manifold bolts to 132 in-lbs using the tightening sequence in the FSM.

One thing I have not done is rented a fuel pressure tester. When I relieve the pressure at the valve it squirts once. Not like a fire hose or anything. Could a bad fuel pump cause this code? It starts right up without any additional cranking or having to pump the pedal, unlike when the fuel pump went bad on my 92. But I don't know what else to suspect right now.

Any ideas/ advice appreciated.

[1oldman]

Wow! You've covered all the bases I could think of with the exception of maybe the ground wire from the ICM Bracket, or maybe the PCM. I'm guessing you'll need a scan tool that does real-time graphing to get a good shot at what is the root cause. I'd say that the only other thing to do is throw parts at it, but that is expensive and time consuming if you don't get the defective part right off. Hopefully someone else can put you onto the cause right away.

Before Electronic Controlled Ignitions, I had the same problem on a new 'vette. It was a hair-line crack in the distributor cap that you couldn't see without removing the cap and giving it a very close inspection. - BC

[zuper8]

Used the fuel pressure tester at Autozone today. The reading I was getting was 5 psi... really??? I'm surprised, it starts right up without hesitation 100% of the time. Followed the instructions with the kit.

There is a nylon line on the car that a previous mechanic repaired with a brass union. It has a very slow seep, the line is wet at the union but it is a slow enough leak that it attracts dirt, it's not bright and shiny. Hard to believe that would be enough to make the fuel pressure drop so drastically, but I'll fix that before changing the pump. This car has 18 New England winters on it, those fuel pump straps and quick disconnects are certainly not going to be cooperative.

[MKMike]

Truly hard to believe it would run at all with only 5 PSI of fuel pressure.
Maybe the gauge was off to some degree?
You're wise to be fixing the nylon line first.
If the pressure truly is that low, then it would seem highly likely that the slow seep would become a steady spray once the pressure is in the normal range with the new fuel pump.
A slow seep isn't going to cause a drastic drop in fuel pressure.
Changing the fuel pump is a lousy job but thinking of the money you save doing it yourself can offer some comfort.
Just don't skimp and save money on a cheap fuel pump. After all, it isn't a job that you'll want to redo in the near future.

[crash93ssei]

I would try another gauge, I very highly doubt the car would run at all on 5 PSI yet alone be able to do any sort of spirited driving. Make sure the gauge line is threaded and seated all the way on the fitting, key on to run pump to see what it goes to, start engine and check while running, remove vacuum line off FPR to check if pressure rises.

[zuper8]

I can see if Advance has one. Autozone has the Actron kit. I cycled the ignition on and off several times before starting the car. It said to do this until the car reaches recommended pressure, but it wouldn't do anything. No pressure whatsoever. So I eventually gave up and started it. I did the whole process twice and got the same result... I agree, it doesn't make any sense, I was like WHAT?? I did not think to pull the vacuum connector off the FPR though.

[crash93ssei]

Sounds like either the gauge / hose was not all the way threaded on the fuel rail or it is a bad gauge.

[Datbonneville1]

I had pretty much the same issue on my 94se, had 15 lbs of pressure at idle, and would stumble greatly under acceleration but idle like it was perfect. Replaced the fuel pump along with the fuel filter, and its been running perfectly since then!

[zuper8]

Ahh, yeah it's looking like that could be the only thing, your experience is helpful too since you only noticed a problem when accelerating as well. I know I had the hose for the fuel pressure gauge threaded all the way on and finger tight, so who knows. Time to suck it up and change the pump this weekend!

[zuper8]

Finally getting the fuel pump in the mail tomorrow, 14 days after I ordered it. Never ordering from Autozone online again. Tried to drop the tank today, as I feared the nuts for the strap bolts broke loose and the bolts are just spinning now. I can't fathom what kind of wrench I could get on that nut, maybe a crowfoot wrench on an extension? I dunno, I think I'm gonna cut the straps and replace everything. I therefore need to buy a Dremel, I can't believe I don't own one of these things already anyway. Will the Dremel 100 series be an ok tool to do the job? It's just and only high speed, which is great because the only thing I ever wanted one for was for cutting steel bolts and the like.

[zuper8]

Replaced the fuel pump, no change. Anyone have any ideas for me? At this point I'm considering paying the Chevy dealer 90 bucks to diagnose it for me with their fancy equipment so I don't waste any more time and money.

[2000Silverbullet]

Have you replaced the Throttle position sensor?

[zuper8]

Hm, I have not, I have a spare I'll try out. The car REALLY acts like it has a vacuum leak, except I can't hear it, see it (I've replaced most of the connectors), and spraying things with brake cleaner doesn't make the idle change. A friend told me that an EVAP solenoid, valve, or canister could be opening up at the wrong time and causing basically an internal vacuum leak? I also replaced the PCV when I did the intake gaskets, I don't know how I could have screwed that up but perhaps I did?

[nos4blood70]

Make sure the PCV valve has the o-rings in the right places, if you try to remove the oil cap, is there a vacuum?

[Roadrunner]

I have had a couple times where I thought I ruled out plug wires, moved on to check/change other parts but then in the end they turned out to be the culprit.
Sometimes they check OK but then move them around a little and not be OK.
Since you already tested them, I would check again by looking for corona with engine running in pitch dark, shift them around a little & check again. Also visually inspect whole wire for bad spots.

With these symptoms I would have also guessed coils and maybe ICM but looks like you are on top of that.
Have you checked all grounds for ignition & fuel pump?

[zuper8]

Fiddled with a few things yesterday while I had a moment to work on the car.

So, no vacuum sound when I take the oil cap off and engine running. I did re-check the PCV valve, gasket ok, O ring is on there, I just pushed the valve in until it stopped, then put the spring back over top and then the cover.

Suspecting an internal vacuum leak, I capped off the EVAP valve and canister, separately, no change. Ran out of time to try anything else.

The main feature of this is that it seems like a small vac leak at idle in park, then it gets drastically worse when I shift it into reverse or drive. I can watch this happen on the boost gauge.

I'm gonna take off the EGR tube yet another time to see if the JB weld is holding up over the crack... and yes maybe another wire test wouldn't hurt. I could see the spark in the daytime when I was testing them but maybe it was too bright to notice subtle differences in the spark.

[MKMike]

The PCV valve should have TWO o rings; one in the bottom and one at the top.
I don't know if it is true of Bonnevilles, but I recall that , years ago, in some GMs, anything other than an ACDelco brand PCV would cause problems.

Believe it or not, a bad battery will cause P0300.

If the EGR valve itself is bad, then it will also cause this problem.
In the old days, a rap with a hammer would get an EGR that was stuck slightly open to close.
If the internal EGR seat is "carboned up", it will not be able to seat.

Hope you'll let us know the outcome.

[zuper8]

Well, the latest development is that the boost gauge stays at 0 or higher, and the supercharger doesn't seem to engage. Also have p0171. And, the car isn't missing noticeably under load, nor is the light flashing. Still have P0300 though. But the car can drive around town seemingly normally aside from not having a supercharger. All the pulleys are definitely turning and the belt is fine. I looked over all the supercharger vac stuff three times and I can't find anything out of place. What massive vacuum leak is this that can't be found?? I need to read up in my FSM on how the supercharger stuff works, maybe that will point me in the right direction.

With a problem this bad, I know I'm going to smack myself on the forehead when I finally find it. It must be something really obvious and I'm just not approaching it the right way.

[zuper8]

The boost gauge is bouncing around, and I have seen it go down when I'm accelerating, and up when I let off the throttle. So something is up with the bypass valve it seems to me. I had to remove it, and all the other stuff, when I was painting the blower this past May. I looked it up in my FSM, and apparently there is actually a J tool to install this thing. Really?? I thought you just rotate the rod until it slides into the lever and then done. Has anyone else used the J tool? How could I have screwed up the BCV....

In other news, I cleaned the ICM ground, quadruple-checked the EGR tube, replaced the vac connector going from the BCV to the BCS, and spritzed the coils and wires with water to see if it would miss. No change. At this point, ever since the boost wierdness, it has stopped missing noticeably whether at idle or under load, but P0300 is still pending and I also now have a P0171.

[2000Silverbullet]

Intake gasket air leak?