cold start, bucking and missing, '89 305cu.

[papiichuulo]

I am working on an '89 model 1500 w/ a 305cu. and a 5-spd. On cold start (mornings) the engine just doesnt want to run. If you ease on the gas/accelerator, it will gently try to get up to speed. But if you try to "rush" her and give her a little more gas, then she starts bucking and missing, backfiring.... that is until you let off the gas and ease back into it. After you run it for about 10 maybe 15 minutes, it'll start running pretty good. We have checked and replaced.... the dist. cap and rotor, ign. wires. sp. plugs, IAC motor (broken spring), temp. sender (cracked plastic) and connector (broken wire)....... I was almost sure that the tune-up with new temp. sender would take care of it, but apparently not... I am wondering about the O2 sensor now.... I dont have access to a pre-OBD II scan tool at the moment.... Any suggestions?

[2000CAYukon]

Which temp sender did you replace? The one in the driver's side head or the one in the intake manifold? The one in the intake manifold is the one for ECM. The other is for the gauge.

Since it is running bad cold, I doubt it is the 02 since the ECM will be in Open Loop.

Check for any vacuum leaks. EGR solenoid should hold a vacuum (TBI side).

With the air cleaner off, how does the spray pattern look at the injectors?

//2000CAYukon

[papiichuulo]

Originally Posted by 2000CAYukon

Which temp sender did you replace? The one in the driver's side head or the one in the intake manifold? The one in the intake manifold is the one for ECM. The other is for the gauge.

Since it is running bad cold, I doubt it is the 02 since the ECM will be in Open Loop.

Check for any vacuum leaks. EGR solenoid should hold a vacuum (TBI side).

With the air cleaner off, how does the spray pattern look at the injectors?

//2000CAYukon

I replaced the sender in the intake manifold. I didnt think it would be a vacuum problem, since it only acts up when cold. Didnt even think about the EGR, I'll have to look into that one. Injectors have a good spray pattern.

[J-Ri]

Vacuum leaks can be temperature sensitive. When everything's cold, it's slightly smaller. As it heats up it expands and can seal the leak.

[papiichuulo]

Originally Posted by J-Ri

Vacuum leaks can be temperature sensitive. When everything's cold, it's slightly smaller. As it heats up it expands and can seal the leak.

I see, (said the blind man to his deaf daughter).... I'll check for vacuum leaks tonight. Thanks for your help.

[diggin]

Originally Posted by papiichuulo

I am working on an '89 model 1500 w/ a 305cu. and a 5-spd. On cold start (mornings) the engine just doesnt want to run. If you ease on the gas/accelerator, it will gently try to get up to speed. But if you try to "rush" her and give her a little more gas, then she starts bucking and missing, backfiring.... that is until you let off the gas and ease back into it. After you run it for about 10 maybe 15 minutes, it'll start running pretty good. We have checked and replaced.... the dist. cap and rotor, ign. wires. sp. plugs, IAC motor (broken spring), temp. sender (cracked plastic) and connector (broken wire)....... I was almost sure that the tune-up with new temp. sender would take care of it, but apparently not... I am wondering about the O2 sensor now.... I dont have access to a pre-OBD II scan tool at the moment.... Any suggestions?

If it's backfiring then for sure it is an electrical problem , try a new coil. O2 sensor will only cause a high idle , you can also look at the egr by disconnectig the hose (plug it and drive it).