I own a 1989 Chevrolet Scottsdale k2500 With the 5.7 Liter 350 TBI. Shortly after buying it the head gaskets blew, So i took it to a mechanic I thought i could trust to have the gaskets replaced. Well ends up having two cracks in each of the heads which was very evident after the machine shop cleaned them up. So in comes two remanufactured cylinder heads. All new valves, springs, everything new. So I get the truck back and its running great, better than ever. For about two weeks... Then one day i'm getting on the freeway and it starts bucking and backfiring like its gonna kill over everytime I try to give it gas it starts acting like its gonna die. The weird thing is it will idle ok but cant go above about 5 mph and it starts acting like it wants to die. Things i've replaced after having all the head work done, Plugs, cap, rotor, whole distributor, spark wires, egr valve, injector pig tail ends, o2 sensor, Volt checked the ign. coil and its right where it should be. Checked the fuel pressure at the TB its where it should be. Not sure what else to do, The mechanic had it for two months and still couldnt figure it out. I know this is a long story but any help would be great. Do the rocker arms maybe need adjusted? Its got good compression all the way around so i'm not sure it would be the rocker arms?

 

check engine light on? have you checked timing? if compression is good, then it wouldn't be rockers.

 

No check engine lights oddly enough, The timing was a little off but has since been adjusted, Thats kind of what I was thinking maybe the timing advance but doesn't seem to be that. What about a vacuum leak? maybe intake manifold gasket? Do you know a way to check the intake manifold without pulling it off?

 

you got cats on it? the whole 5mph thing sounds like the clogged cat but you saying backfiring means theres flow enough through pipes to get pops. so, you seeing any loss in coolant? white smoke? funky smells? anything else different?

 

I actually pulled the whole pipe off thinking it might be clogged. I just left the manifolds and y pipe and it still did the same thing. Only coolant loss seems to be a bad thermostat housing gasket which I replaced. No funny smells or any smoke at all. very clean exhaust, visually anyway, and I guess it smells normal like exhaust would. Thanks for any help, Could my throttle body maybe be plugged somewhere internally? Just enough flow to idle but not enough the accelerate?

 

The Backfire part definitely sounds like timing. Were the reman heads you put on the same as the one's removed? Same springs valves and rocker arms. If not were the clearances checked on the pushrods ect? Could the clearances have been to tight and it ran good until something bent or broke in the valve train?
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As far as the vacuum leak spray some carb cleaner around the intake and listen for a raise in idle speed. Check the torque on the heads and intake bolts.<O</O
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I'm not sure if clearances were checked or not i'll have to look into that. Thanks for the tip. I assume they were being that it was a mechanic doing the work but you never know. Thanks again.

 

So while the truck was down and running like $#!& I replaced the water pump cause it had been previously leaking and I knew it had to be done, what better time? Put new water pump in and a new "serpentine" belt. Fired it up today just to make sure there were no antifreeze leaks. Well it ran perfect for about twenty minutes, then the check engine light came on and it started acting up again. I got a code 42, which is Electronic spark timing, open or grounded EST or bypass circuit. Does this mean my timing is off? Like I said the truck ran like brand new for about twenty minutes. Idled perfect and had more power than its ever had, I thought it was fixed for sure till the light came on.

 

For those of you having gm obd1 with code 42 and having a GM pre 1995 vehicle and getting the check engine light in your dash, please read carefully below. (For GM vehicles made before 1995 that has a distributor type ignition and has code 42-EST circuit failure and has the following symptoms:
Stalling at stop signs
No start but if you wait up to 2 hours, it will start again
TESTS for GM obd1 with code 42:
1.) Please confirm this gm code first by resetting the code and see if the code will show up again a second time. You reset obd1 codes by disconnecting the negative battery terminal for 1 minute and reconnect. If the code shows up again, it means this code is really a hard code that must be fixed. During no start, confirm the presence of ignition problem by checking the presence of spark using an especial tool ST125(cost under $5) and available in any local parts store.
2.)Make sure there is 12 volts supply going to your distributor harness wire when you turn the key on engine off (KOEO).
FIX for GM obd1 with code 42:
Remove the distributor assembly and replace the following:
Pick up coil
Ignition module
Once the above parts are replaced, reset the code and go for a quick road test to confirm. If your engine does not have a distributor, replace the crank sensor instead of the pick up coil. But check also for input voltage to the crank sensor harness wire. In some cases, the crank sensor magnet probe could be cracked and make sure to check them by removing the sensor for close visual inspection.
When checking the components for gm obd1 with code 42, you are better off using a wiring diagram or a vehicle service manual. This way you can avoid removing the wrong part. This is especially true when you try to identify the electrical components using the colors of the wires which are shown in the wiring diagram. If you need assistance, contact ATS.

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I see above that you replaced the whole distributor. Was it a brand new one?