Tahoe Power Problems

[LI Tahoes]

Hello all. I have a problem I was hoping GM Tech or someone out there might have some input for.

 

For background, I have two Chevy Tahoes. A '99 that is my wife's a a '98 that is mine. Both have the 5.7L V8 and four wheel drive (4 door if that matters).

 

The '99 runs great and has no issues. It drives great and, when necessary, it pulls right up to close to 5000 RPM and shifts firm.

 

The '98 is another story. The '98 has adequate low end power, but accellerating onto the highway or applying heavy throttle creates problems. Anything over about 50% throttle is very weak. At 75-100% throttle, the engine will run up to about 3000-3500 and just kind of linger there, never approching the shift point. The only way to achieve upshift is to back off on the accelerator to lower the shift point.

 

Troubleshooting this far has been all the basics. First, there are no codes in the computer and the service light is not on (except for bulb check at startup). I have done a complete tune-up, including wires, cap, rotor, and platinum plugs. I had marginal fuel pressure (high 40s PSI) so the Fuel Pressure Regulator has been replaced (with new upper intake gasket and O-rings) and the pressure is up to specs. Also, I have watched fuel pressure under acceleration (when the drop off occurs) and it stays rock solid). Mechanically, the drive-line has no binding or excessive reistance.

 

I had marginal compression at just over 100 all around, so I recently did a timing chain. I now show close to 115 all around. Thus far, the symptom has remained the same. I swapped the intake duct (with MAF and MAT sensors) from the '99 and reset the computer to see if that would help. No change. I swapped the ECU from the '99 to try and troubleshoot. Still no change (I put the ECU's back into their respective vehicles).

 

I am at a loss for the problem here. The fact that everything appears to check out good and the computer cannot find a fault has be baffled. Any ideas?

 

Something I found in the Alldata DIY site has to do with the Crankshaft angle learn procedure. Is this possibly it? It sounds like if this is off, it will have premature fuel cutoff (which is what it feels like). If it is, can this be done at home, or is this a dealer only type of procedure?

 

Any information or though would be greatly apprecated. I am new to the board and have already learned a lot just browsing through some of the posts.

 

Thanks.

Dennis

[ChevyTech007]

You have covered most bases. The next thing I would do is a back pressure test on the exhaust system. Seen many partially stopped cats cause your exact problem, and it is very easy to over look.

[LI Tahoes]

Thanks for the feedback. I have had a quiet suspicion about the cats. Are there any published specs for backpressure? I have the (mostly useless) Chiltons for this truck, as well as an AllData DIY online subscription, but I can't find anything that tells what backpressure is acceptable for each side.

 

Also, is the Crank Angle Learn something worth pursuing, or is it not likely to be an issue?

 

Thanks again.

 

Denns

[GM-Tech]

There are specs for backpressure. I don't remember them exactly but it should be under 1.5 psi at 2000 rpms. On most good systems I see NO pressure reading at idle and barely a pound at two grand.

 

It certainly won't hurt to do a crank relearn, but it ain't gonna help with your problem.

 

Compression is low.

[LI Tahoes]

Thanks again for the feedback.

 

What is good compression for a '98 350 Vortech? Since I did my timing chain, I have been getting about 115 all around (within 1 PSI) with WOT and plugs out. I thought this wasn't too bad, since the only spec I could find was alldata which said the minimum was 100 PSI with the lowest no less than 80% of the highest.

 

100 (or 115 even) sounded low to me, but I had not thought it a problem in light of the "100 minimum" spec. What is good compression for these?

 

Thanks again.

 

Dennis