Tbi 350 Idle And Miss Woes

[awalkertx]

Hi all,

 

I've had this truck for 10 years now, and over the past 6 months it's developed two problems that i'm not convinced are related. It has the L05 TBI 350 and MG5 manual transmission. ~200k miles. I love this truck, and if the engine didn't have good compression, i'd have already transplanted an L31 into it.

 

MISS PROBLEM(started a month ago): Engine will miss under a load above 2000 RPM. It is clearly be seen on the tachometer... the RPM will jump up 500 to 1000 more and then drop to zero. I replaced the plugs, wires, cap/rotor, coil, pickup coil and ignition control module... engine timed to 0 degrees TDC per the sticker on the fan shroud. The truck acceleration is considerably better (and my AM stations work without a ton of RPM-dependent interferance); HOWEVER, the problem still remains. The miss is less pronounced but still there... more like it will jump +/- 500 RPM when the truck goes over about 2200 RPM.

 

IDLE PROBLEM (began in the spring): The truck when cold will start up fine and high idle (~1100 RPM on the tach), then warm up to a normal idle (~700 on the tach). If driven a while and then turned off, then restarted shortly thereafter, the engine will die unless I keep some right pedal in... It's like it doesn't even want to try to idle. I can drive it around at this point, but more often than not need to keep a little gas pedal in when I come to a stop. I can partly alleviate the idle problem by inducing a vacuum leak (pulling the large vacuum tube off the font of the throttle body), which is no way fixing the problem. EGR valve is only a few years old, and acts fine when depressed and the truck is cold. I had a MAP HI code a few years back, but that's no longer being flagged.

 

any ideas? thanks for any insight!

 

Adam

 

PS, I don't want to add that it has an Edelbrock MPFI conversion on it, which has been fine since 2001 and uses all of the original sensors and throttle controls... looking for a more basic issue/fix before I swap the old stuff back on.

[awalkertx]

UPDATE: Idle air control valve fixed the idle problem. Replaced the throttle position sensor for good measure.

 

The truck still misses when under a load up over 2000 RPM. It doesn't happen every time I drive it, but probably every other time. I've already replaced pretty much all of the ingnition parts (cap, rotor, plugs, plug wires, coil, pickup coil, igniton control module). Any other ideas? Again, thanks for any input anyone might have - I'm pretty frustrated!

[Dean_o]

Sounds like you might have a wiring issue. Check all the grounds that you can find.

[dietz4ibanez]

Actually I would check the gear on the bottom of the distributor. Mine was missing very minimal at first and got worse over a period of months. When it was taken out some of the teeth, well at least half of them were worn down to the width of a razor blade. I was going to look into the cam but the gear is a softer metal which wears a lot easier!

[awalkertx]

roger guys - will check both.

 

i did pull the distributor out when i replaced the pickup coil. i din't look closely (or even directly) at the distributor gear- i hope i would have noticed an issue. i've no doubt missed bigger things though!

 

with the grounding issue, we're talking wiring harness right? this would lend credence to the fact that it is intermittant.

 

thanks a ton!

[dietz4ibanez]

let us know what you find out!

[Dean_o]

Yes, basically any grounds you can find. Most will be pretty easy to find under the hood of the truck and under the dash.

[awalkertx]

Yes, basically any grounds you can find. Most will be pretty easy to find under the hood of the truck and under the dash.

 

 

found the culprit. bad ground at the thermostat housing stud. the "sandwiching" nut had backed off slightly and the two wires there could move freely. cleaned and tightened - now the truck runs like a champ. i think it's running better now than it did 50k miles ago...

 

THANKS A TON!!!