charchri4

OK I know this is way off topic but I'm going nuts trying to figure this out and need another set of eyes.

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

He 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 we 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?

He picked up a spark tester and will check for good spark tonight and verify the fuel pressure at all RPM ranges it will do. Other than that I am stumped on what the problem could be. Any help would be appreciated.

V8droptop

I wish I was there, I could probably help you on this. Never done a longitudinal 3.8, but dozens of transverse ones.

Sounds as if it could be vac related, as in unmetered air. When you open up the throttle and it acts like it does, kind of stumped as well, how that would cause this symptom. You can try a smoke test or using carb cleaner/etc around suspect areas, and see if the rpms change.

Other than that, is it the original cat? Have you checked it with a soft rubber hammer or something? DON'T HIT IT HARD, but you can gentley tap or shake it to see if it makes metalic noise. The other give away is driving at night and seeing the exhaust glow infront of it. I'm putting this here because I've had fuel issues cause cat's to die, for instance, low fuel pressure = running lean = too much heat in exhaust, possibly damaging the catalyst's core. Had a bad coil ran too long on a car kill a cat, as well as a few other things.

Like I said, wish I was there, I could probably get you closer faster :-(

Cheers,
Nick

V8MiataMike

Does it have/use a mass air meter? sensor dirty/covered/clogged? Maybe throttle position sensor?

Will it idle indefinitely?

charchri4

Thanks for the response I am going nuts here!

Yep same basic set up as the LS1s. I have tried unplugging pretty much ever sensor there is and it doesn't change how it runs just toughs codes. The catalytic converter rattles but not the hollow hard rattle of one broken up. I could open up the exhaust and try it but it seems like a real stretch that it would be bad just from running 15 miles on low fuel pressure.

He double checked his spark on all 6 wires with it tached up as much as it good and the spark gets brighter as it starts to die so I'm kind of thinking the spark is good. It's got a ultragauge so he can watch timing and it does advance as the rpms go up.

He also double checked the fuel pressure tached up as far as it would go too and it goes from 42 psi at idle to 50 as it tachs up then stays there as it bogs out till death.

Yep it will sit there and run all day just fine as long as you put no load on it at all. I'm going to have him post up a video I think.

charchri4

Gator Bait

sure sounds plugged up... refusing to die is interesting though... swap in a known good ecu? CPS known good?

charchri4

I was about to the point of thinking ecu. I'm going to have him drop the exhaust tomorrow and check the cat and see what it does with open exhaust. I heard a lot of hissing in the video to so I'm going to have him pick up a can of carb cleaner and go hunting for vacuum leaks. This thing is going to be the death of me yet!

tbone heller

Have you checked the air filter?

Try dropping the exhaust & see what that does.

charchri4

Yep he pulled it out and ran it with out as a test. But that is a really good thought and helps to point me where to go. I think we have done everything we can to prove spark and timing is good so that leaves air and fuel.

It dawned on me on the way to work that we should look at the fuel trims to see where they are at especially when it's bogging down. That might give a clue to what is happening.

For air in and out tonight's plan is to drop the exhaust and verify the cat is good and get a can of carb clean to look for vacuum leaks.

I posted this on LS1tech and a couple F body forums and have gotten way more useful ideas here than there!

MX-Brad

My dad had a turbo'd t bird that did the same thing. Clogged cat. Also did some other damage iirc due to the bad cat.

charchri4

The thing that bugs me about a clogged cat is it ran perfectly up until the fuel pump died. So for it to be a cat it would have to have clogged it up driving it that 15 miles on low fuel pressure and I'm not seeing how that could happen. If it damaged it or melted it some how that I could see but you would think that would be from being too rich not too lean.

But there is no question it sounds like it is gasping for air and that is a pretty easy thing to check on this car. 2 bolts splits it in front of the cat and we are going to find out tonight.

Thank YOU!

Dave Tibbitts

When it had the old pump out, did you check the voltage at the pump connector? It could be something as simple as a faulty fuel pump relay. Did you drop the tank to change the pump or did you make a trap door for access?

charchri4

LOL oh boy would I be kicking myself stupid if it was something that easy but a really good point. He did the trap door and it's still open so would be easy to check. We do have solid fuel pressure though. It sits at 42 at idle and goes up a tad with RPM and stays there as it bogs out. So we can prove pressure and put a new filter in it but we can't really prove flow.

V8droptop

We are all on the edge of our seat. It doesn't take much to kill older cats, I don't know why but I've had 3 GM vehicles and an Audi melt them down in short time. Having a failed coil does it quick, but running lean it will be the heat that melts it. I honestly don't know the temps/conditions, etc, I just usually see the aftermath. They work just fine and are no issue till something else goes wrong, and then you have a cascade of issues from your initial problem.

Still wouldn't hurt to check compression, btw, but I'm doubting that if it idles as decent as it does.

BTW, have you pulled the intake pipe off at the TB and checked for anything broken? Dunno how/what, but choking for air, I'd follow the air intake tract from the filter back. Can't be much harder than some work with a screw driver.

Good luck, I'm hoping the Cat comes up the issue, easy enough fix.

charchri4

Well the cat guys win the prize! It was welded on but I told him he 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!!!

Dave Tibbitts

Glad you got it sorted. One less thing between you and getting your Miata up and running. How's it coming along?

Gator Bait

The second you count out a possibility w/o testing you ensure its going to be the issue. Glad it's sorted. Likely was on it's way to clogged and just got finished off during the lean condition. I'd do a complete tune up b/c like you figured...that 15 minutes wasn't enough to kill it alone imo.

V8droptop

Could be, Michael. But Cats, especially on older vehicles are really sensitive to heat and inproper fuel mixture. I've killed one from excess fuel, but seen more than a few melt. The honey comb mixture at the front starts to melt and goes back into the rest of the structure and you end up with a big bunch of molten metal plug.

We have similar issues with our Diesel Particulate Filter ( DPF )and SCR (diesel version of cat) on heavy equipment. People kill them with all sorts of stuff, but heat makes quick work of the ceramic/precious metal matrix.

chpmnsws6

Ls swap. /thread!

charchri4

LOL the car doesn't need more motor just a lot less car!

So any reason the feel he may have damaged anything else along the way here? I thought the plugs looked OK but should he change them maybe?

V8droptop

Never hurts to check the systems over, if he did enough damage to kill the cat, I'd check ignition/compression. Other than that cross fingers and give it a go.

memory serves, our 3.4 went through a stuck valve, cat, and coil all at the same time. One begat the other in that case.