Flat lobe(s) on the camshaft maybe? gm had problems with that years ago. Hadn't heard any problems with something this new though.

Whatever damaged the catalytic convertor the first time is probably damaging the new one, causing it to melt and clog.

Maybe stick a vacuum gauge on the intake manifold to check for an exhaust restriction, like a clogged catalytic convertor?

A suggestion after re reading your post. Pick one place and stick with it. You're starting over every time this way. Your complaint is poor fuel economy and power loss. After someone does a diagnosis and repair, and it's still the same, take it back to the same guy. He'll know what he did already and move on. Any place I worked a mechanic customer wouldn't get charged full bore for every appointment chasing the same complaint.

I did not always have all the answers either - right off the bat, even with 8 years experience, and the vehicles of today are far more complex and difficult to work on.

A man needs to have the intellect of a brain surgeon at times - and these people are smart enough to be doctors and surgeons, not mechanics..The car-makers do not help at all with poor manuals and training programs and primarily vehicles that are so difficult to diagnosis and repair..
As Gary says, stick with one man, even if he is not successful the first time.
Patience is a virtue..

for awhile g.m. used double walled exhaust tubing, dont know if they stil do or not, but when a catlytic converter went, the heat caused the internal wall of the exhaust tubing to collapse causing a restriction, that made the car so it wouldnt accelerate. you might want to have this angle looked at.

It sure seems like there is an exhaust restriction some where. A vacuum test may give a clue. Worn cams or lifters can cause this problem.

Other possiblities is that something got unplugged during the service work, either intentional or by accident, and was not reconnected. This could be a sensor or a vacuum hose. A motor may start, but idle poorly if the MAP sensor is disconnected. If the oxygen sensor signal is missing or grossly out of bounds, the engine controller ignores it and uses a default setting. It's on the rich side.

And finally, has anyone checked the ignition timing? It may show ok under no-load, but goes bonkers when loaded.

By the way, an exhaust restriction can happen anywhere along the line. It can be present between the catalytic converter and the end of the tail pipe. It's unusual in this location with the usual being holes. but if there is a resonator down the line, check it out. Also, closely examine the entire exhast line.

Watch out for loss of, or out of bounds temperature sensor signals. Many systems have two temperature semsors. One is for the temperature gauge on the dash, and the other one signals when its time to turn on the electric fan. The engine controller uses a temperature input.

Fianlly, bad electrical connections/wires can give fits because these are hard to find and can cause false information to the engine controller.

If you have run too long with a restricted exhaust, the engine oil becomes comtaminated early and allows parts like cams and lifters to wear out early. A restricted exhaust casues the vacuum to drop to subpar values (under load) and the PCV system can't properly purge the crankcase. Fumes back up and escapes through the air inlet back towards the air cleaner. A common symptom is a an oiled air filter. But of course, worn rings or leaky valve seals on the exhaust valves can do the same thing.

Thanks for the suggestions. After returning from the 4th shop, I've decided to stick with ONE mechanic to solve the issue.

A co-worker whom has a brother-in-law in the business told me that most larger shops and dealers wont spend much time diagnosing problems as a matter of policy. Mostly because there is no money in it. They normal don't charge anything if they don't fix anything or replace any parts.

Form what I read, I need to get the exhaust system thoughly tested. OK...will do.

It was also suggested (again by a co-worker) that I get a Dyno Test done by an independent shop, and use these results to help pin down the issue. Worth it?

Also - It was suggested (aren't co-workers helpful?) that my issue may be a sticky valve, or something in that order, and it may be a good idea to use 93 Octane fuel for the next couple tank fulls to clean out some of the snot in the fuel system, and to get my oil replaced with full-synthetic oil for added protection until the issue is resolved.
Good advise...or Waste of money?

At this point, I'll stand on one leg and recite the star spangled banner if it would help.

When the exhaust is restricted, even a little, the engine operating temperature will increase, the top end will be lower, starting will be harder.. All of this depends on the amount of clogging..
Not sure in how to test this without guessing or doing service work...

Any shop is wrong for not doing a proper diagnosis and the necessary service work AND charging accordingly.
And the time and cost for this is usually not that great..
Nor is it right for anyone to expect a "free diagnosis"...

I would not bother with the expensive premium gas or the synthetic oil.
I'd guess that the reason why the original catalytic-converter failed is due to inexpensive construction - this will happen to any car...If the reason for failure was an excessive amount of unburned gas; then this cause must be fixed..

So you have two of them, what kind of shape is the second one in ???

A non-functioning oxygen sensor will increase fuel consumption by 10% or so, I would think.

Your MPG is off by 50% !

**Also - It was suggested (aren't co-workers helpful?) that my issue may be a sticky valve, or something in that order, and it may be a good idea to use 93 Octane fuel for the next couple tank fulls to clean out some of the snot in the fuel system, and to get my oil replaced with full-synthetic oil for added protection until the issue is resolved.
Good advise...or Waste of money?**

Waste of money. If you had a sticking valve it would be noisy. I suggested cam wear because it keeps the valves from working the way they should, but does not make any noise. Power loss is gradual till you have one or more nearly dead cylinders. Exhaust restriction is another possibllity. Premium fuel won't do a thing to solve your problem. Synthetic oil won't do a thing to solve your problem. No cheep tricks this time. You need a good diagnosis and repair.

Hi Tim....

Your message became quite a celebrity in and of itself on our professional site for the automotive industry, www.iatn.net

The industry has changed a lot in the past few decades and not all of those changes are under the hood of your car. :)

A well-respected shop owner/mechanic posted a link to your message and several responses were curious as to the details of each visit you made to each of the 4 repair facilities...

It is my personal opinion, as the owner of an independent repair facility (meaning I have no contract with nor allegiance to any auto manufacturer) that your situation is not abnormal.

The vast majority of the dealer network mechanics work on what is called flat-rate, which basically means they are paid a certain amount of time for each job, regardless of how long it actually takes the mechanic to complete.

Suffice it to say, this puts jobs that require diagnosis (like yours) at a disadvantage...when a mechanic needs to eat (which is often), he or she may tend to shy at job that pays .2 (12 minutes) when it can easily take 2-3 hours of diagnosis/test driving to find the problem. They never recoup that time. Their paycheck will be smaller that week. So the easy out is to write up "no problem found". And your vehicle is shipped, unrepaired.

While I cannot diagnose your actual vehicle over the internet, I can suggest several things.

1. Find a reputable mechanic and stick with him/her. For life, if possible. You develop a relationship with a mechanic just like you do with other service providers, like hairstylists, veterinarians, etc.

2. Find a good basic mechanic who is ASE certified, perferably MASTER AUTO with an L1 certification. Certification is voluntary in our industry, so anyone can claim they are a mechanic, but someone who is ASE certified has shown themselves to take pride in their work and has volunteered (and paid) to take tests to prove their mechanical knowledge. The MASTER means they are certified in all 8 areas of auto repair, the "L1" designation means they are certified in advanced engine computer electronics...it's sorta like a doctor being able to fix the harder cases...

3. Find a mechanic from www.iatn.net This group is probably the more knowledgable of the industry, and although there are bad apples in every basket (which my horse will eat ANYWAY, he'll have you know...) you're more likely to find someone local you can trust and develop a relationship with.

4. Avoid "chain" repair stores. (I'm going to get slammed for this "back home", but oh well...) They typically pay lower wages and employ some of the less experienced mechanics in our industry, although there are always exceptions. The stores themselves are geared towards "sales" which means they are slanted towards installing what they sell and not much other types of repairs. You might get a sweet deal on tires or shocks, but they might flounder somewhat when faced with a problem that involves intricate diagnostics like your own problem...

Sort of like buying furniture for an odd sized room...you can buy mass produced cardboard stuff that is available in only one size and color, or you can find a local cabinet maker to make a beautiful dresser that fits your room's odd dimensions perfectly. At this point, it sounds like you need a cabinet maker....:)

Here is a link that might be useful: international Automotive Technician's Network

Hi Timgren

Yes there are still real mechanics, although we think of ourselves as Automotive Technicians these days.

As has been said by others, you need to find a good shop, with a strong diagnostic technician, and then stick with them even if they don't nail this the first time. However, after what I write here, if I don't skip anything I should be able to give them all the ammo they need to get this one.

One of the things that I have preached for the last 5 odd years that I have posted here is for people to test, and not just throw parts at someones car problem "because that is what was wrong with their neighbors, or relative, or whoevers car" when it was doing the same thing. Todays cars simply have so many ways to produce the same symptoms that nothing but solid diagnostic routines will provide accurate answers. Sadly, the industry still tries to compete between shops by price, and consumers reward this by trying to go to the cheapest place, and hopefully for them the cheapest fix first. Thats why you have had plugs thrown in, filters, a fuel pump ect. Most of the time the symptoms you describe can be caused by these components, but not always. One of the most telling parts though was the convertor replacement. That is something that has two possible causes for the failure, either a missfire was occuring and or the convertor overworking, or quite commonly insulation between the double walled pipe in front of the convertor is coming out of place and blocking the convertor. Without knowing first hand, I'd have to rely on seeing your truck to know, but I suspect you have more than one convertor. One of the things that need to be done is a road test with a good scan tool, (GM's Tech II) not some $300.00 code reader and a snapshot of scan data saved to be looked at to analyze the failure. Basically if the engine is losing power something is affecting either, fuel delivery, spark timing, spark strength, engine displacement, or some combination of those. Scan data has the ability to show a trained technician almost all of that except for strength of the spark.

What I would do if your truck came here is first road test the truck under the conditions that display the loss of power and capture a snapshot of the scan data. When I got back to the shop I would concentrate on; mass airflow sensor readings in grams per second, MAP sensor voltage if used, engine RPM, throttle position, spark timing, knock sensor, timing retard, fuel trims, and O2 sensor voltages. One of the things that I can do back at the shop is calculate the volumetric efficiency of the engine using the MAF sensor reading at a given rpm. This would tell me right away if the engine is breathing correctly. I would compare MAF readings while the problem is occuring to TPS and MAP readings. BTW during the symptom event, I would purposely move the throttle around a bit, while keeping the truck acting up and then examine the results on the scan data. Fuel trims and O2 sensor voltages would tell me if we are dealing with a fuel pressure, fuel volume problem. Knock sensor, spark timing, and spark retard could help me identify everything from simply a bad knock sensor, to mechanical engine trouble, to even something loose on the truck rattling and making the knock sensor send a signal that the computer see's as the engine pinging.

Lastly while back in the shop, I would go back over secondary ignition and ensure strength of spark, and other normal issues. Off hand I expect you have an individual coil for each plug. That pretty much in itself rules out a number of potential problems that could occur with other designs that could cause your symptoms.

This testing that I have outlined would take very little time and would either have me already knowing what is wrong, or it would have me narrowing the investigation to only a few points having ruled out a large portion of the operating system. One thing that is mandatory is to forget anything else has been done to the truck, and test exactly as it is presented, and allow the results to lead me to the problem as it exists today.

Feel free to contact me by e-mail once you have this back to a shop if they have trouble figuring it out. I will give you contact information for them and can help them analyze this via live chat on the net if they want.

John G. ASE CMAT L1

BTW, thanks Akthy. Take care, we got to get together one of these days. Maybe Arts up for another party....

Ok, Timgren, here is another thought on this 8 mpg,top speed of 60 situation..

There is nothing really wrong !!

The accelerator cable has become too "long" or has slipped out of place (which I would think to be impossible)..

The fuel economy is not being monitored correctly - the electronic read out is "wrong" showing but 8 mpg ..
Maybe there is a connection between the two problems...
The traditional method of filling up, recording the gallons and odometer reading is much more accurate,IMO, - but it does take a while to establish meaningful numbers.
Very few people fill up 100% every time, nor do they fill up to the same exact level every time..
And it is unreasonable to expect this, particularly with the heat, cold and rain factors...
So it is much easier to rely on the the cars computer (and I do this as well) ...

And computer systems NEVER makes mistakes do they ???

Anyway, Timgren, this would be a very unusual situation..

But these newer vehicles do have computers and "fly by wire" so different and unusual things do and will go wrong, as always..

Thanks John for the insights in proper diagnostic proceedures. That's what concerns me most about the first post. Didn't anyone at any of these places verify the complaint or do some kind of diagnosis? Seems strange. Shouldn't be that hard to run down the problem if they follow some kind of proceedure like you said.

As to dealerships and flat rate, when I worked in those places I would prefer this kind of work. Warranty flat rate pays a lot less than customer pay flat rate. Sounds like this one is out of warranty so I'd be waving my hands at the service writer hollering ME ME ME to get this one.

Hi Gary. Great questions...

>Thanks John for the insights in proper diagnostic proceedures. That's what concerns me most about the first post. Didn't anyone at any of these places verify the complaint or do some kind of diagnosis?

I'd have to say there had to be at least some testing at some of them, but not complete testing. The sad part is why it's rare to get complete testing. Shops respond to the demands made by consumers over time.

Case in point,

1st senario: a customer calls and describes a problem. Shop gives an estimate based on the description, customer approves it and comes and has that work done.

2nd senario: Customer calls and descibes a problem. Shop quotes a diagnostic fee, and outlines what testing they will perform in order to identify the problem and then estimate the repair.

Which shop do you think is busier? Which shop do you think gets the repair correct the greater percentage of the time?

The public by and large support the first shops attempt because they percieve it to be "cheaper" and they feel they can save money. When the reality is for some of them it is cheaper, and for another group it is many times more expensive.

The second shop that charges correctly actually has trouble attracting new customers that start with a "how much is it" phone call. What this all leads to is every shop isn't equipped, and it's people trained to do diagnostics at certain levels, and it's because they are a slave to market pressure. It really gets tough to hold the line when a lot of customers act like they think your trying to take advantage of them by charging for diagnostics when so and so down the street dosen't. But ultimately we end up at Timgren's problem The shops that allow market pressure to determine their diagnostic stratagies cannot handle Timgren's truck's problem, be they an independent or a dealer. Like I said it's funny in a way, people get what they pay for, and eventually and in his case now, he's not getting what hasn't been paid for for decades..

>Seems strange. Shouldn't be that hard to run down the problem if they follow some kind of proceedure like you said.

That's a relative thing. As I outlined it isn't hard when the tech has the tools, training, and experience to carry the routine through. All kidding aside, if I didn't have his problem nailed in 1/2 to 1 hour I'd be very dissapointed with myself. Granted we often see cars that develop half a dozen problems over a couple years that the owner simply ignores until one problem becomes so bad they are forced to get it into a shop which makes for a very difficult diagnosis. I have walked up to some cars and found five legitimate causes for a single symptom. How do you truly fix the car unless you correct all of them? It's always great to be able to go up to a car, and diagnose a single part as the source of the trouble. A tech has to not get lost in the myriad of possibilities that can be presented.

One of the first parts of diagnosis is "listening to the customer"..
No easy task at all as most people have trouble with relating "mechanical ills".
On the other side, all too often ,the listener is not allowed sufficient time to listen to everything in depth, so much is lost.
We must get away from "hurry - hurry- hurry" mode in which many are stuck..

This is why I bring up the throttle cable being stretched or out of adjustment..
And , do we know for sure if the mileage is being properly measured - by man or machine...

Hello timgren, This happened to me a few years ago. I had a '89 Honda Accord that started losing power badly, no accleration, never checked the gas mileage. I took it to a Honda dealer thinking maybe bad valves. The mechanic called back asking my wife if we had a dog and if the dog was losing weight. She replied yes and no and why did he ask that. He said he had found almost a pound of dog food in the breather tube and in the breather itself. We kept the car in the garage and also fed the dog there. A mouse or mice had been taking the dry dog food and storing it in the breather and tube. The mechanic saved the found food in a plastic bag to show us. I couldn't believe it but there it was and the car ran fine after that. I put a screen over the breather tube and never had any more problem. Might be a good idea to check your breather and tube. It might even save you some money.