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Posted by gblotter (75.37.22.237) on 23:46:30 07/29/08
In Reply to: Follow-Up to: She's Just Getting Old posted by Marty
Marty - you may be on the right track by suspecting your cat.
Here is a re-posting of a nightmarish experience I had back in 2006. Sounds similar?
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Well, after 8 weeks and $6300 in repairs, my mechanic called today to say that my Suburban was ready to be picked up. They added 290 miles in road testing during those 8 weeks. My head is still spinning from the shock of it all. Both of us took a bath in this deal. My mechanic only billed a small fraction of the actual hours he spent diagnosing the performance problems.
In the end, I believe it was a multi-layered problem that began in August 2004. Here is what I ~THINK~ happened.
August 2004: Observed dramatic drop in fuel economy (11.5 MPG highway) during roadtrip from N.California to Victoria B.C. Blamed it on new tires with high rolling resistance, and also rooftop cargo box. I now believe more was going on. Beginning symptoms of faulty EPROM that was retarding timing and causing rich fuel mixture.
February 2005: Continued poor fuel economy during roadtrip to Disneyland.
July 2005: Narrowly avoided interior fire in Suburban due to catalytic converter getting so hot under the vehicle that it melted wires against the floor board. Smoking, smoldering mess of melted wires almost caught my Burb on fire. Added insulating blanket to underside of vehicle as remedy, but that does not explain the excessive heat. Blamed heat on new header-back Banks Powerpack exhaust system. Now I believe the cat was running so hot because of the fuel rich condition had to burn off the excess gas. Fuel rich condition from undiagnosed faulty EPROM.
August 2005: Encountered backfires during roadtrip to Yellowstone. Blamed it on bad gas. Continued poor fuel economy. After taking Burb to the dealer for repair, they found timing to be way off and reset the timing to within specs. Backfires damaged catalytic converter, but I didnt know it at the time. I now believe that backfires were caused by rich running condition. Backfires, rich fuel mixture, and bad timing ultimately the blame of undiagnosed faulty EPROM.
October 2005: Encountered unusual loud clicking sound and sluggish performance during roadtrip to Disneyland. Thought engine was going to implode with bad valves. Only happened once while climbing a steep grade. Symptoms vanished as fast as they appeared. I now believe it was another manifestation of bad timing from faulty EPROM. Continued poor fuel economy during trip.
April 2006: Continued poor fuel economy during Spring Break roadtrip to San Diego.
July 2006: Continued poor fuel economy during roadtrip to Park City, Utah. Experienced near total loss of power - suddenly occurred while driving at freeway speeds in hot weather. Vehicle nearly disabled on interstate. Limped vehicle into nearby GMC dealership. Dealer found bad timing and replaced complete distributor assembly and reset timing ($800 diagnosis/repair). Performance still bad, but good enough to drive vehicle home at 50 MPH. During trip home, encountered steep grade. When I punched the accelerator, sudden loud hissing sound started and continued for the rest of the trip home. I speculate that blockage in catalytic converter somehow breaks free thus the dramatic, sudden performance improvement and also hissing sound. The next morning, the hissing sound is gone (pieces in catalytic converter cooled off and shifted around?). Performance is acceptable for in-town driving.
July 2006: Took vehicle into same performance shop that originally installed the Banks Powerpack header-back exhaust system (including hi-flow cat). Explained scenario and requested that they check/replace cat. Mechanic reports that exhaust system pressure test is fine, but timing is way off. Long, expensive diagnosis ordeal begins. Checked/tested everything from fuel pressure, vacuum, air intake, alternator, timing chain, various sensors everything is within specs. Ultimately decided to replace PCM, but performance falls on its face in even more dramatic fashion than before. Timing will not advance past 20 degrees, but that does not explain dramatic performance fluctuations. With advice from this forum, decision is made to also replace EPROM. Mechanic makes recommendation to use Hypertech ThermoMaster Power Chip. With Hypertech, timing advances properly and baseline performance is significantly improved. But performance still falls on its face in dramatic, sporadic fashion. Believe that it is somehow heat-related. Mechanic notices rattling sounds from catalytic converter and loud exhaust. Decides to retest exhaust system pressure. Pressure check looks bad this time. Decision made to replace catalytic converter (8 weeks after I suggested the same). With new catalytic converter, performance failures vanish.
It is good to have my Suburban back home again. Actually, with the new Hypertech chip, the performance is better than it has ever been in the past. Now it really hops when you step on the accelerator. That is probably a combination of the Hypertech and the Banks Powerpack exhaust system. I could never see the expected performance improvement from the Banks exhaust system before because of the faulty EPROM issues that were masking/overriding any benefits.
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