Low idle at start up 87 350 tbi. update, backfiring?? last post

[Jessie James]

The shop manual says:

On engines with TBI, spark timing changes are determined electronically by the ECM which monitors information from various engine sensors, computes the desired spark timing, and signals the distributor

So if you have advanced the timing, the ECM doesn't know that and is possibly causing your backfiring.

Did you disconnect the vacuum line from the distributor (if equipped) when you timed it?

Sounds to me you should put the timing back to the factory spec and take it from there.

The other thing that may be an issue is knock. If you have advanced the timing, when the ECM advances the timing more, then knock may occur, and in that case the ECM will retard the timing to prevent engine damage. That will also reduce efficiency and performance.

 

[4xcrazy]

The shop manual says:

So if you have advanced the timing, the ECM doesn't know that and is possibly causing your backfiring.

Did you disconnect the vacuum line from the distributor (if equipped) when you timed it?
.

Not on a computer controlled TBI engine.... It's that one little wire that gets unplugged for timing..

 

[eagle mark]

Not on a computer controlled TBI engine.... It's that one little wire that gets unplugged for timing..

It's really hard to read your posts with that avatar...

 

[mudboggink5]

Backfiring through the intake can be lots of things like a vacuum leak, bad intake valve or running lean. If you think about it theres no reason to give an engine full throttle at idle in park.neutral so maybe it's not programmed into the ECM even though most engines can do this.

Since you have other issues can you put a vacuum gauge on the engine? Look at idle vacuum, what is it? Is it steady?

What should the vaccuum pressure be?

 

[mudboggink5]

The shop manual says:

So if you have advanced the timing, the ECM doesn't know that and is possibly causing your backfiring.


I didn't touch the timing. I'm gonna borrow my buddys timing gun and check it out.

 

[eagle mark]

What should the vaccuum pressure be?

Depends on your elevation.

But around sea level 20 inches of vacuum and goes down as your elevation goes up

The shop manual says:

So if you have advanced the timing, the ECM doesn't know that and is possibly causing your backfiring.


I didn't touch the timing. I'm gonna borrow my buddys timing gun and check it out.

You'll have to find the timing disconnect wire first. It is usually a tan wire with black stripe and has a conecter. Disconect it. Then start truck and set timing to 0 and tighten distributor. Connect wire and restart. This usually causes a code 42 if you check codes afterwards. Won't hurt anything but disconect the batterey for a minute to clear.

Not sure what manual says advanced timing may be causing a back fire through the carb? But it is true the ECM will not know that you set the timing wrong. Biggest problem there is if you set timing to say 5 degrees advance the ECM does not know and when total timing advance comes in it will be 5 degrees to much timing for that motor and cause pinging/knock.

 

[LT280Z]

Not on a computer controlled TBI engine.... It's that one little wire that gets unplugged for timing..

OK, im back with results from all the comments in this post. I had a very similar problem to the OP's and his post described my truck perfectly. I read the post's and determined timing was the first area i would check since timing a motor properly greatly effects the way a motor starts. Removed the brown wire and set it at 0, reconnected and drove it for about 20 minutes or so. PROBLEM SOLVED! woohooo!!!

No more code 43 or code 45 and the truck cold started great every day since Christmas. I just rebuilt this whole motor and outfitted it with new components so i was pretty sure it was an adjustment rather than sensor failure, etc.


The motor's base timing was rediculous and im suprised detonation didnt get the better of it after seeing the base timing. No wonder my throttle response was so aggressive The timing was so far advanced it was off the teeth/timing bracket. Talk about a MPG improvement

THANK YOU INJECTION SECTION!

 

[mudboggink5]

good to hear. i fixed my low idle problem, and the truck runs much better. It still has that hesitation during low exeleration. Timing will be the next thing im gonna check. Just need some time.

 

[TexasSilhouette]

Hesitation, backfire, and hard starting are all classic symptoms of low fuel pressure.

 

[GMC Truck]

Just curious but do you still run the heat pipe from the exhaust manifold? This was missing from my truck which ran great untill it got cold out and then the low idle hesitation started untill its completly warmed up. My GM dealer told me to replace that missing tube to the manifold and wala! That fixed it plus way faster warmups!!

 

[eagle mark]

Love my heat risers in North Idho. Truck runs as good at 0 as it does at 100...