Engine Hesitation/Stumble at WOT

[SVOeric]

Blue - thanks appreciate the reply. By chance did you check the fuel pressure? The thing that I don't understand is decrease in fuel pressure when the boost stays constant.

this means the fuel pump can't keep up...

[BlueCASVO]

My fuel pressure is set to 39 at idle, and climbs to 55ish at 16 lbs of boost, and stays there. I am still running the dual pump setup, not the single in tank setup.

If your fuel pressure is dropping gradually under boost, I would think you have a problem with your fuel pump (check valve, or not enough voltage) or the FPR. I am assuming you have already checked for gas in the FPR... Do you have a voltage gauge that you can hook to the car while driving? Make sure you are around 13 - 13.5 volts in the system at WOT. When I had the charging problems with the 85, at WOT it was 11.4 volts - not enough to power the fuel pumps, ECU, and ignite spark under 16 psi, which caused misses, backfires, stumbling, all kinds of strange things.

Clean stable power is essential to keeping a boosted engine running right...

[Felung]

I will check the voltage and report back. With the 3g it is putting out 14.5 at idle and revving in driveway. The replacement fuel pump arrives tomorrow and I will change that in the next few days.

One question - and I am searching for this answer now - do I need to drop the gas tank to do the fuel pump change? Update: I did more searching and found that replacing the stock in tank pump on an 86 is a fairly simple task - so am looking forward to getting this complete and will do an update after.

[NavySVO]

- do I need to drop the gas tank to do the fuel pump change?

Yes. I would get all new seals on hand before you start the project.

[BlueCASVO]

Yes. I would get all new seals on hand before you start the project.

Why does he need Navy seals to stand around while he changes a fuel pump?

Have a GREAT NAVY day!!!

[Felung]

Just making sure I have everything needed before I start. When you say all new seals, are there more needed than just the fuel pump, which I am asssuming bolts in and requires a gasket?

[NavySVO]

Why does he need Navy seals to stand around while he changes a fuel pump?

Have a GREAT NAVY day!!!

Bwa-ha-ha-ha.....and it's SEALs (an acronym gets all caps)

Just making sure I have everything needed before I start. When you say all new seals, are there more needed than just the fuel pump, which I am asssuming bolts in and requires a gasket?

You will need;
A (square) o-ring for the pump bracket, new fuel-compatible hose from pump to bracket nipple, and the filler neck doughnut. 50resto has a kit with all this stuff. You may need a pump sock too. It depends on the parts company you went with for the pump.

The fuel pump gets fastened to the bracket. The bracket has a round, flat plate that the supply and return lines run through. The o-ring goes between the plate and the tank. The bracket is held in place by a ring that is spun into the tank flange. You will see 2 tabs that are there to assist in the installation/removal of the ring. It only turns like 3/4 of a turn or something like that. I recommend using a non-sparking drift to avoid the gas tank from becoming an IED.

[NY eric]

Bwa-ha-ha-ha.....and it's SEALs (an acronym gets all caps)


You will need;
I recommend using a non-sparking drift to avoid the gas tank from becoming an IED.

Aw come on that takes all the fun out of the job. on a serious note spraying the lock ring down and letting it sit a while is recomended especcialy if the car spent any time in the salt belt.

[Felung]

With any luck I might make the Darwin Awards on this one! More seriously, thanks, will get the kit, "soak the salty nuts " and hopefully get this thing sorted.

[Felung]

OK, on the bright side, I replaced the fuel pump without ending up on the Darwin Awards (that I know of).

The symptoms were exactly the same so I turned the boost down to 9lbs, unplugged the knock sensor and it ran perfect. I guess I will be buying a knock sensor next or figuring out why it thinks there is knock.

[Jcamper]

I would like to throw another idea out. I chased a very similar issue around and around with my car. Turned out to be weak valve springs. This works into your latest "fix", also, as turning the boost down would resolve it. Mine would act up right at the same rpm yours is, and it was super consistent, would do it every time. Jeff

[Felung]

JCamper: After reading your note I ran the car with the boost turned up to 16# and the knock sensor connected and the engine stumbled again and then disconnected at 16# and the engine ran at is should. I am very happy it does not appear to be the springs

[Turbo8u]

Felung, did you ever get this fixed? I'm having a similar issue now.

[Felung]

Yes

Two issues came up, Knock Sensor and the Auxillary shaft. Try the Knock sensor first!

[ns3racing]

1984 with 70014 miles.

My car to has what appears to be the same issue. Short bursts with boost ok. When I climb the mountains here in Oregon and the boost stays up, it stumbles and has now power. Do you know if the 84 model year has the same knock sensor? And also how hard to swap this out. I have chased the problem for a year now.