99 Tahoe driveability

[biggsteve97]

Have a 99 Tahoe 350 4wd with about 250k on the clock. Runs rough, lathargic and very rich. Have replaced all four oxygen sensors, plugs, wires, cap, rotor, ignition control module, ignition coil, mass air flow sensor, fuel filter, and air filter. Everything done has improved things but the problems are still there. Getting between 4-6 mpg MAX! Only codes from the computer have been oxygen senor rich codes. Also the engines idle is barely 500 rpm in both park and drive. Im thinking of changing throttle position sensor next. Also tryin to find live data reader to see what the computer is seeing. Any suggestions? Im also thinkin that maybe the spider assembly might be leaking profusely but havent had a chance to pull upper intake because the vehicle is driven daily. Cant afford the fuel costs.
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[STOCKISH]

With a scan tool connected, what does the live data show for the coolant temp sensor. What temp is it reading? When they fail they can read negative temps and richen the fuel mixture to help warm up time. If it stays negative, fuel will always be way rich.

[biggsteve97]

Originally Posted by 1972SuperCheyenne

With a scan tool connected, what does the live data show for the coolant temp sensor. What temp is it reading? When they fail they can read negative temps and richen the fuel mixture to help warm up time. If it stays negative, fuel will always be way rich.

Unfortunately, I don't know yet. Have not been able to find a live data reader. i only have a code reader and they guy I know that had a live data/code reader sold his to purchase some high priced data reader device he will not loan out. Trying to convince him to meet up and take a look at it now. We have acquired both a temp sensor and the mpfi conversion spider. I am considering worse case scenario, installing the temp sensor first and see if that fixes it, if it doesnt then i will pull the upper intake and inspect the spider. Only solution I have been able to come up with at the moment unless i can get the guy I know to let loose of his scan tool or bring it over. ugh, I soo wish I could afford to buy one.

[speedygonzales]

My first inclination is the spider. The first MPFI spider I got was no good. Fuel pressure was too low but that's another story.

Your diagnosis should have used a fuel pressure gauge to see what normal operating pressure is and if it stays up after the engine is shut down. First and fore most.

Also with that many miles on the clock and running rich, your cats are shot. You should not have wasted money on the after cat O2s without looking at the cats that we know now are shot. Pull them apart and see if it runs a lot better.

Good luck

[biggsteve97]

Originally Posted by speedygonzales

My first inclination is the spider. The first MPFI spider I got was no good. Fuel pressure was too low but that's another story.

Your diagnosis should have used a fuel pressure gauge to see what normal operating pressure is and if it stays up after the engine is shut down. First and fore most.

Also with that many miles on the clock and running rich, your cats are shot. You should not have wasted money on the after cat O2s without looking at the cats that we know now are shot. Pull them apart and see if it runs a lot better.

Good luck

That was my first thought as well but its not my truck. I also havent been able to spend much time diagnosing it. I would have checked fuel pressure but I never remember to take the fuel pressure gauge with me when I go pick up rent from them. Most of the time the vehicle is not there when I am over there and he has not brought it to me so its his own fault now for it taking soo long to fix. I didnt know until right before my first post that he had changed all four O2 sensors but thats what he gets for not listening. I am very worried about the cats now as well from it running soo rich. Besides the black smoke seems like it is more restricted per say. Not as much force/air pressure if you can call it that at the tailpipe. under acceleration the smoke really doesnt have much authority coming out the pipe anymore.
I will find out more tomorrow. Hes finally agreed to bring it to my house and leave it for 3 hours.

[speedygonzales]

Remember the after O2s are only used to check on the condition of the cats. they have no effect on fuel ratio.

[biggsteve97]

Ok. Well he showed up with tahoe at 11am this morning and we were done at 6:30pm. Imediately after poping the hood noticed some rubber cap on a heater hose port on the radiator that was severely swollwen and cracked. After removing the air cleaner assembly found a severely rotted vacuum hose for the heater valve and over 3/4 of the length was missing so it had a vacuum leak there. Found the heater hose port on the intake cracked, leaking and the oring for the tube trying to push its way out of the slot on the side of the connector. Previous owner had tried some kind of leak stop. Found it caked on the temp sensor and what turned out to be the factory thermostat that was stuck open. It was severely low on coolant so my thought was combined with the gunk build up on the temp sensor it wasnt getting a good reading there. Then we decided to go ahead and open the intake. On the way we found mis routed vacuum lines and loose bolts. Someone who didnt know what they were doing tried to attemp this repair is what we were thinking. Fuel lines were not attached to the bracket on the back of the motor either. Seal was torn on the spider spider as well. Once inside we found atleast an 1/8 inch thick build up of carbon and oil on the intake floor, in the runners and the back side of the upper intake. Most of the idle circuit was closed off by carbon build up. The backside of the throttle body was built up thivker than the blade support is high. Couldnt even see the butterfly suppot it was so thick. Also the runner directly below the fuel pressure regulatot was completely clean and i mean clean shiny like it was new and so was the regulator and one of the injectors on the bottom of the spider was veryy clean. That injectory was also pushing the retainer that holds it into the spider out. My guess is it had a severe fuel leak from the spider and the regulator. After replacing the spider with the conversion spider, the temp sensor, thermostat, and fixing the coolant and vacuum leaks the engine runs awesome now. We do notice a hesitation around 3800 rpm and higher so we are thinking the cats are damaged from the severe overfueling. A dual exhaust system is coming soon anyways so the cats are going to be replaced no katter what. I will post pictures of the intake soon. Mobile phone will not let me. I must say of the five spider conversions i have done this was by far the nastiest and most carboned up of all of them and the worst running prior to the repair. And the only that didnt have the service engine light on.
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[biggsteve97]

We had started cleaning the intake floor a little just before the pic was taken. We scraped gunk out of the intake for atleast an hour. Huge chunks were down in each runner. Wish I could have gotten a pic of the upper manifold to show how thick the carbon was especially on the throttle body.