I recently rebuilt the TBI 350 in my '87 K5--all stock except for flat top (versus the factory dished) pistons bored .30, balanced, and a Holley 670 cfm TBI unit. It runs great, except the check engine light is constantly on, giving a bad knock sensor readout on the diagnosis. The sensor is new and hooked up right, along with every other sensor. The engine also stalls slightly when I take off from a stop and occasionally when the tranny shifts. I thought it may be that the computer doesn't like the small increase in compression, but I am not sure that is it because it never pings (even with the cheap gas). I am at a loss for what to try next--maybe someone out there has had similar trouble? Thanks for the help. [email protected]
CK5
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EFI engine trouble
- Thread starter herg193
- Start date
MousePowrd
1/2 ton status
So if you disonnect the battery to clear the ECM, reconnect it and start the truck the light comes on immediately, or it doesnt do it until you get on the gas?
And if you run premium gas (a full tank, not mixed, and not octane booster) do you get the light?
And if you run premium gas (a full tank, not mixed, and not octane booster) do you get the light?
D
**DONOTDELETE**
Guest
I have an 89 K5 Blazer with a 350 TBI and I had a bog down on the bottom end when I would take off from a stop. After spending around $350 in parts chasing the problem, I did a voltage check on my pick up coil in the distributor and it was only putting out .06mV when a good one should be putting out 1.0 mV. Do a voltmeter check on your pick up coil, it could be the culprit.
Later,
Doug
Later,
Doug
chillywilly
Registered Member
Its possible the knock sensor might be picking up a very slight piston or valve train noise.Then its telling the pcm to retard timing. Might be able to insulate the sensor a little with some plumbers tape.Just keep an ear out for spark knock if u try this. /forums/images/graemlins/thumb.gif
Or, you can get a manual and a scanner, and see what the knock counts are.
Thunder
3/4 ton status
I dont know all the specs for your engine. But putting in flat top pistons will raise the compression ratio more than just a little. If you went up just one full point to around 9.5 to one you are pushing the limits on the stock chip.
It is set up for an 8.2 to one engine. They will do ok at 9 to one or a little higher.
When you raise the comp your engine needs more fuel to keep it from pinging. If it pings(even though you cant hear it the knock sensor can) The knock sensor tells the ECM to reduce timing. It will keep reduceing timing in 4 or 5 degree steps till the pinging stops. If it doesn't stop at a programed point, it sets a the Trouble Code.
BTW Is your EGR dissconnected? That can also cause problens on a high compression engine. it needs the EGR to reduce combustion chamber temps. This helps stop pinging also.
You really should find out what your compression ratio is and get a chip burned for that ratio.
Your other problem may be the POS Holley TB. I have always had nothing but trouble with Holley FI. The stock TB unit is WAAAAY Better! And will flow more than enough Fuel/Air for your engine.
It is set up for an 8.2 to one engine. They will do ok at 9 to one or a little higher.
When you raise the comp your engine needs more fuel to keep it from pinging. If it pings(even though you cant hear it the knock sensor can) The knock sensor tells the ECM to reduce timing. It will keep reduceing timing in 4 or 5 degree steps till the pinging stops. If it doesn't stop at a programed point, it sets a the Trouble Code.
BTW Is your EGR dissconnected? That can also cause problens on a high compression engine. it needs the EGR to reduce combustion chamber temps. This helps stop pinging also.
You really should find out what your compression ratio is and get a chip burned for that ratio.
Your other problem may be the POS Holley TB. I have always had nothing but trouble with Holley FI. The stock TB unit is WAAAAY Better! And will flow more than enough Fuel/Air for your engine.
