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I have a 1999 Tahoe vortec 5.7. It has a somewhat rough ...
Sent to Car Experts March 19 01:52 PM

I have a 1999 Tahoe vortec 5.7. It has a somewhat rough idle when the engine warms up.   It also will throw a ses light when pullung a long grade. the code is always p0300: multi-cylinder miss. I have had the fuel injectors replaced, and later cleaned, but it still does this. The dealer says that they cannot reproduce the problem.

Customer (name blocked for privacy)
Status:  Closed   Value: $30
Answer
March 19 2:12 PM (19 minutes and 36 seconds later)
         
REPLIEDCheck Mark
It could be fuel pressure related.I would check the fuel pressure and replace the fuel filter if it has not been done recently.I have seen where this only occurs below freezing temps and a couple bottles of dry gas in the tank can help.You should have 60-66 psi key on engine off.You will need to cycle the key several times to check it.It shouldn't go below 55psi with the key off for 1/2 hour either.

Edited by diablo666 on March 19 2006 at 2:14 PM

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March 19 3:46 PM (1 hour and 34 minutes and 31 seconds later)
         
Reply to Davo/diablo666's Post: I bought a pressure tester from pep boys and it reads 56 psi. I changed the filter and it still reads 56 psi. While the fuel pump is running, it reads 60 psi and drops to 56 when the pump shuts off.
Customer (name blocked for privacy)
Answer
March 19 4:02 PM (16 minutes and 7 seconds later)
         
REPLIEDCheck Mark
Then the fuel pressure should be okay.How many miles are on it?I want you to also give this a try.Remove the Mass air flow sensor and spray carb cleaner or brake cleaner in it to clean off the sensing wire.You may be going slightly lean if it's too dirty.You don't have a K and N filter do you?

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March 19 4:22 PM (19 minutes and 38 seconds later)
         
Reply to Davo/diablo666's Post: 95,000 miles.This problems has been going on for about a year now.(15k-20k miles) Sprayed it with carb/choke/tbi cleaner. Still runs the same. No k&n filter. I have the stock filter. (Looks like the type used on forklifts)
Customer (name blocked for privacy)
Answer
March 19 4:28 PM (6 minutes and 4 seconds later)
         
REPLIEDCheck Mark
It's possible you have what is called carmaling of the valves.Deposits form on the valve stems and cause them to hang open at times.If you have a vacuum gauge you can check it pretty easy.Hook the vacuum gauge to the engine at manifold vacuum(vacuum all the time)and see if the needle bounces rapidly during the misfire.If it does you have some type of valve problem it could even be weak springs.I do hear about the carmaling popping up more and more lately because of the changes in gasoline.It may be partly due to the fact that the EPA has required reducing sulpher content in gas.This has caused new chemicals to appear in gas that were not there before,sulpher oxides is one.

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March 19 4:32 PM (4 minutes and 12 seconds later)
         
Reply to Davo/diablo666's Post: I checked that yesterday. The vacuum gauge read 18 inches at idle. It didn't move at all. When I hook my diagnostic computer up, it reads 10 inches at idle.
Customer (name blocked for privacy)
Answer
March 19 4:34 PM (1 minute and 21 seconds later)
         
REPLIEDCheck Mark
So the needle doesn't move during the misfire?

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March 19 4:35 PM (1 minute and 9 seconds later)
         
Reply to Davo/diablo666's Post: Let me re-check. I do not remember any movement.

I re-checked. The needle does not move. It reads 18.5 inches today. In gear, it reads 16.5 inches. The engine still has the rough idle. When I say rough, it sort of shudders occasionally. Not really bad, but bad enough to notice and have people tell me I should get it fixed.

Edited by Customer (name blocked for privacy) on March 19 2006 at 4:45 PM
Customer (name blocked for privacy)
Answer
March 19 4:55 PM (20 minutes and 3 seconds later)
         
REPLIEDCheck Mark
You will never get rid of that little occasional quiver it is present in almost every one of those engines.It may be worth pulling the distributor to see if the gear on the bottom is worn.It can cause a misfire.Match mark the rotor and distributor position before you pull it so you can put it back in the same place.

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March 19 5:02 PM (7 minutes and 43 seconds later)
         
Reply to Davo/diablo666's Post: The distributor was replaced when the injectors were replaced. The main part of my question is the problem pulling grades. The service engine light comes on everytime that I drive over our local mountains. The one time that it did not, I was only going 50 miles per hour. Usually though, traffic is going about 70mph. One time, on a local hill, the car was running really rough, the service engine light came on and the engine immediately smoothed out. That was the time that I took it to the dealer and they cleaned the injectors. It did run better, but the light still comes on in the mountains. It always comes on blinking; then stays on solid. It eventually goes away, sometimes after a few hours, sometimes after a day or two.
Customer (name blocked for privacy)
Answer
March 19 5:20 PM (17 minutes and 23 seconds later)
         
REPLIEDCheck Mark
Tape your fuel pressure gauge to the windshield and go drive the truck on that hill when you get a chance and look at the fuel pressure when the misfire starts.

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March 19 5:29 PM (8 minutes and 43 seconds later)
         
Reply to Davo/diablo666's Post: Sounds like a good idea. How much fluctuation is allowable if any? Is 56 psi acceptable at speed like it is at idle or should it be in the 60-66 psi range?
Customer (name blocked for privacy)
Answer
March 19 5:37 PM (8 minutes and 32 seconds later)
         
REPLIEDCheck Mark
It should be up around 60-66 when pulling a hill closer to the high end is best.56 probably is too low under a load like a hill.The rule of thumb is higher pressure for higher speed and higher pressure with increased load.In other words the further you push the gas pedal the higher it should go.

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March 19 5:39 PM (2 minutes and 2 seconds later)
         
Reply to Davo/diablo666's Post: I am assuming that if the pressure is too low, the fuel pump needs to be replaced. Is there a differece between aftermarket parts and dealer parts?
Customer (name blocked for privacy)
Answer
March 19 5:47 PM (8 minutes and 20 seconds later)
         
REPLIEDCheck Mark
I would say if you buy a quality aftermarket pump you would be okay.Make sure that if you replace the pump you also replace the harness that connects at the top of the pump.It will be a four wire harness.If you don't change it a repeat failure may occur due to improper terminal contact.There is a bulletin out about that and I still see other techs in my shop just throw a pump in the truck.Two of them stopped by as I was replacing a harness on Friday and asked are the wires bad?I said nope I just like changing them because I can read can you?

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March 19 5:57 PM (9 minutes and 10 seconds later)
         
Reply to Davo/diablo666's Post: Is the harness a dealer only item?
Customer (name blocked for privacy)
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March 19 6:21 PM (24 minutes and 7 seconds later)
         
ACCEPTEDCheck Mark
No I bought the one Friday froma NAPA store.

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