charchri4

I have a 98 Firebird 3.8, 5 speed 105,000 miles as a DD. The fuel pump failed but pumped enough to let me drive about 15 mph for a few miles before it died totally.

I put a new pump in it and it starts right up now but does pretty much the same thing. It won't run past 1500 RPM and if you go to wide open throttle it will slowly bog out and die.

What I have done and know:
No codes
42 psi of fuel pressure and it holds it for 5+ minutes after shut off.
Fuel pressure regulator was new 5,000 miles ago
Tried a new ignition module no change
Replaced the fuel filter but it was fine.
Pulled 5 of the 6 plugs after revving till it dies and they are not wet with fuel and no sign of anti freeze



Plugs and wires were new 2 years ago at 80,000 miles
Timing does advance from 10 to 30 when I tach it up.
Compression in #1 is 145 and I can't really get to the rest of them.

It runs a lot better with the oil filler cap off so I'm concerned there might be some mechanical damage in it?

I picked up a spark tester and will check for good spark tonight but I am stumped on what the problem could be. Any help would be appreciated.

Injuneer

Try resetting the PCM (pull "PCM BAT" fuse for 30 seconds). If the fuel pump was failing, the "learned" long term fuel corrections could be adding extra fuel to compensate for the low fuel pressure. Resetting the PCM clears the long term fuel corrections.

charchri4

I thought that too and dug into it quite a bit. I did have the battery out of the car for 2 days doing the pump so it's cleared. But near as I can tell until the 02 sensor has enough heat to report to the computer it runs from a programmed set of values based on temp, TPS and MAP.

Thanks for the feed back!

shoebox

Clogged cat?

charchri4

I'll give her a hammer tonight but it's ran perfectly for the last 6 months with a good romp to red line on most pay day Fridays so I kind of doubt it.

Thanks for the input!!!

Injuneer

Three things have to happen before it goes into closed loop - 1) O2 sensors reach 600*F, 2) Coolant temp reaches programmed value, usually around 140*F, 3) timer from startup, usually about 205 seconds, times out. The O2 sensors are 4-wire heated and reach operating temp before the other two events.

In open loop, the only thing the PCM does is ignore the O2 sensors. But, it still uses the stored long term fuel trims. It just can't change them based on O2 sensor feedback. It's still basing air flow on MAF readings, not defaulting to speed-density. The PCM richens the A/F ratio and increases the idle speed at low coolant temps.

High back pressure on the exhaust will cause increased piston ring blowby beyond the ability of the PCV to handle it, which will pressure the crankcase. Removing the oil fill cap would relieve the pressure.

charchri4

Wow that is really good stuff Fred. I hand not considered that the crankcase pressure could be caused by back pressure in the exhaust but that makes a ton of sense. I'm going to check that out tonight too. THANK YOU!!!

I read that if you unhook the battery and hold the positive and negative cables together for a few seconds it will discharge any capacitors and clear the short and long term fuel trims from memory in the computer. It sounded like a wives tail to me, have you ever heard of that?

Injuneer

No.

charchri4

Check this out

shoebox

If you are hearing new strange noises from your exhaust manifolds, then I still would consider checking the cat. The noise could be from back pressure.

charchri4

The shields rattle because he took them off to get at the plugs but it's not too hard to drop the cat so that would be a worthy next step.

charchri4

Well the cat guys win the prize! It was welded on but had to cut the pipe to find out and sure enough it tached right up and had all the power it had before. Why driving 15 miles with low fuel pressure killed the cat is beyond me but sure glad it's running again. THANK YOU THANK YOU THANK YOU!!!