Ok, I have spent considerable amounts of time trying to diagnose a bad running condition in my car (1991 L98 vehicle). At this point, I feel the next step is to start doing irreversible things, which is why I want some more brains to look over everything first.

The issue: The car went from running fine to not revving over 4k rpm in nuetral and not over 1200 rpm in gear, bucking and sputtering the entire time. This was over a 1 week period. At first, it seemed to be misfiring and down on power above 4k rpm, then it steadily got worse until its present state above. Car idles near normal, with no apparent misses.

What I have checked:

1. Bypassed MSD box and coil with a known good stock coil.
2. Pulled and inspected all plugs, all the same color, no problems.
3. Compression check. Motor was rebuilt 11k ago, compression check reveals absolutely no issues.
4. Checked all plug wires with multimeter, all good. Found one that was slightly burnt, no other issues.
5. Checked cap, rotor etc. No issues.
6. Checked timing. Spot on and steady.
7. Checked all vacuum lines/gaskets etc for vacuum leak. No issues.
8. Checked fuel pressure. No issues. Key on immediately makes 40 PSI, with zero bleed down. WIth engine running, fuel pressure does exactly what it is suppoed to do, with no change. verified that motor sputters and conks out while fuel pressure still reads absolutely normal.
9. Hooked up laptop and ran Craig Moates. MAP sensor read normal, O2 read normal, although it did not seem to ever get on the rich side with voltages. NO CODES. All sensors showing normal values.
10. Pulled the valvecovers off, and readjusted the rockers. Made especially sure to look for broken springs, or other issues. FOund nothing, and wasn't expecting anything as the compression test was ok.


So, at this point, I am positive at this point that the motor mechanically is ok, and the ignition, spark and computer are operating properly. Feel free to poke holes in that.

The only thing left that I can think of is a clogged cat. The cat has about 10,000 miles on it, when I converted to shorty headers and a 3.5" Y pipe with a catco cat.

The only thing is...

1. I have seen bad cats before, but in my experience, they go bad much more gradually, and the end result is not as dramatic. I've seen cars that can't run over 2k rpm with a clogged cat, but this car will not runb over idle in gear.

2. Cat is not glowing red, or any other telltale.

Because I can't come up with any other ideas for what is causing this, I am contemplating drilling a hole in the Y-pipe before the cat and seeing what that does. This is obviously irreversible.

So,

1. Can anybody point out anything I missed?
2. Does this sound like a clogged cat?

 

Stick a vacuum gauge and/or watch the map value as you slowly open the throttle in park to see if it starts losing vacuum. You could also remove the o2 and put a low pressure gauge there and see if it builds pressure.

 

Losing vacuum would be a decrease on the MAP value, right?

 

high kpa is less vacuum

should be around 100 key-on engine-off

 

Besides your cat, I would also inspect the EGR system.

 

Have you looked at the possibility of having some bad fuel injectors? Just a thought!!!!!!!!!

 

No way. No code 32, and there isn't any way a bad egr would cause this much of an issue.

 

I don't think so. AT first I thought that was it, but this is much more than a dead cylinder, and if an injector was stuck open (how they commonly fail) I would see it with fuel pressure dropping at Key On Engine Off.

 

You wouldn't see it if the injector wasn't firing (not opening at all).

 

Right. Then I would just have a dead cylinder, and this is running MUCH worse than that.

 

Don't overlook the EGR valve. Mine was stuck open and I had no codes. Car ran like crap.

 

Ok, map value with key on engine off is 79 kpa. Keep in mind that the vehicle resides in Colorado at 6600 feet elevation. I am assuming this is the cause for the lower "base reading".


With engine at idle, KPA reads in the mid-upper 30's.

With the engine revving in nuetral, at first, I see a drop in the KPA value, followed by steadily increasing values.

At the ~4000 rpm point where the motor starts really bucking and shaking, KPA reads in the upper 60's to low 70's. So almost atmospheric pressure.

I take it this is indicating a clogged cat?

 

Just curious, but have you checked the fuel filter?

 

Yes. Thats what all the fuel pressure checks were for.

 

It probably is the cat, just go bang on with a hammer.

 

You know, the more and more I think about it, the more I am leaning towards a bad cat.

Suddenly I remember hearing rattling noises when the car was idling, and meaning to check for a broken hanger... Which could have been the cat breaking up before it decided to fail.

Other peoples experiences seem very similar, driving down the highway with everything ok, then suddenly can't go above 40 mph, then getting worse and worse...

Still I would like somebody to tell me those KPA numbers look off...

 

It definitely sounds like a converter that has seen better days. Now you just need to see what caused it to have a melt-down.