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Old 08-04-2009, 08:41 PM
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Default 99 4.3L hesitation after starting

Well here is my problem. Back about 6 months or so I had a problem when I first started the car and moved forward it would hesitate badly until it cleared out and then ran fine. It was like when the float was leaking on a carb, after first start it would run bad until the fuel was all burned up from the fuel flooding the engine. I had no check engine lights and plugs were just changed 10K previous. I had to change the oil lines so I also put a new coolant temp sensor in and do a coolant change at the same time. Local parts store guy told me they sell lots of them and sounds like it could fix the problem. I didn't think so but since I need the coolant change, what the heck. The problem has continued but would clear up and run fine after the first minute.

The other day I changed the shocks and decided to do the CMFI upgrade. The plenum was really carboned up (115K) as was everything else in there. Cleaned it all out using carb cleaner and rags. Throttle body throttle plate was really bad also. Installed a new Standard brand Fuel injector and gaskets, all went fairly well.

Started it up and no fuel leaks but ran rough because of the carb cleaner. The initial problem is still kinda with me. Now, instead of just hesitating, the misfires a couple of times and then everything is just fine. Again, this happens ONLY on first startup of the engine and not again until it is shut off and restarted and then only sometimes. the engine seems to have a lot more pep now but this hesitating and missing is driving me nuts. No engine codes or service lights. No vacuum leaks that I can find, everything seems fine other than startup. Fuel Pump pressure with key on - engine off is 62 psi then drops to 58 psi when the pump shuts off. After about 45 mins it drops to about 50 psi.

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Old 08-04-2009, 09:19 PM
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Unhook the battery for about 30 minutes, then hook it back up. After a few drive cycles, it should straighten up. If it was gunked up enough, the PCM will have to relearn the intake settings with the better airflow. If it doesn't get any better, post back and we will go from there.

You really should have used throttle body cleaner, not just standard carb cleaner.
Brent Evans

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Old 08-05-2009, 05:24 AM
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I've got the same thing, on an engine with less than 5000 miles on it... So I don't think it's a carbon build up issue...

Make sure your distributor cap and rotor are in good shape...

My truck's had random hesitation and powerloss since day one (and it's on the second engine, lol).
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Old 08-05-2009, 08:56 PM
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It only does it on startup. Lasts about 30 seconds and then is fine. Lots of power and no misses. When I shut it off and restart, it runs bad for about 30 seconds and then is good.
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Old 08-06-2009, 09:06 PM
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Unhooked the battery for an hour. Hooked it back up and took it for a 60 mile ride. After the initial start and missing for 30 secs it ran great. Mileage computer said averaged 21.2 mpg which is pretty good. Lots of power, more than before the injector change. I shut it off a couple of times and restarted but had the same hesitation and coughing for about 30 secs and then fine.

Checked Fuel pump pressure again, Key on-Engine off = 62 psi then lowers to about 56 or so. Engine idling = 55 psi and when I accelerate it pressure jumps to 62 and then back down. At 1000, 1500, 2000 & 2500 RPM the pressure remains at 53 psi.

Vacuum is 19 in at idle and at 1500-2000 RPM it goes to 20 in. When I accelerate it goes to 23 in and then back to 19 in.

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2000, 43, 43l, 99, car, chevrolet, chevy, engine, fuel, hesitating, hesitation, missing, pressure, s10, silverado, start, v6

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