TBI question

[Chickntrk]

Working on an 89 TBI 350

It runs fine, starts right up, goes up and down the highway no problem.

The issue: every now and then (seems like mostly when it’s first ran after sitting) when you pull out from a stop and start to accelerate it’s like a dead spot. It doesn’t cut out or miss it just seems like it’s not accelerating almost like if a carb was getting way more fuel than it could burn and bogs down…then if you back out of the throttle it catches itself and actually accelerates. Truck is a 465 4 speed and it seems to happen mostly during spirited acceleration in 3rd and 4th gears.

Thoughts

 

[Bent77]

That sounds like a fuel delivery issue

 

[Chickntrk]

Since it seems to mostly happen before it’s up to temp I’m wondering if it’s going full rich for some reason and can’t burn the fuel fast enough

Fuel pressure is 13 psi

 

[Bent77]

Pull a plug

I guessing you don’t have an obd1 scanner

 

[Chickntrk]

You guessing right, I am not getting any blink codes though

 

[Bent77]

Ignore that

What we really want from the scanner is sensor voltages and fuel trim numbers

Always been at this pressure?

 

[Chickntrk]

pressure is consistent as far as I can tell I’ve checked it a few times at idle and under load

I will pull plugs tomorrow and post up some pictures

 

[gmbellew]

be worth putting a multimeter on the TSP and working the throttle to make sure the voltage varies from 0.5-1.0 at no throttle to 4.5-5v at WOT with no dead spots or jumping.

EGR issues can cause off idle acceleration issues also. you can test it with a mityvac or just disconnect and plug the vac line between the solenoid and valve and see if it gets better.

 

[Chickntrk]

Checked TPS, voltage is good, I initially thought that was my issue.

EGR has been capped but I didnt remove the valve and put a tin block off plate under it like I have done in the past, so its possible the valve is stuck partially open and still allowing flow.

 

[Bent77]

Two things that came to me


If it’s sucking O2 into the exhaust, it will add fuel to try and burn it
The coil could be laying down

 

[gmbellew]

Checked TPS, voltage is good, I initially thought that was my issue.

EGR has been capped but I didnt remove the valve and put a tin block off plate under it like I have done in the past, so its possible the valve is stuck partially open and still allowing flow.

I'd check for vacuum leaks and maybe swap in a new Delco ICM and see if that makes a difference. Always good to have a spare ICM anyway.

 

[dyeager535]

Checked TPS, voltage is good, I initially thought that was my issue.

EGR has been capped but I didnt remove the valve and put a tin block off plate under it like I have done in the past, so its possible the valve is stuck partially open and still allowing flow.

Depending on valve design one test is to reach up under it while running, and see if moving the valve caused the engine to stumble.

I'd say EGR is fairly unlikely culprit. If the valve was stuck open, you'd have symptoms across the board. EGR being open is basically a vacuum leak, while adding non-combustion supporting gases to the intake.

 

[Wes Harden]

The ECM might still be compensating the fuel strategy expecting the EGR gases in the mixture.
This would make things a tad fat. How low longvhas EGR been plugged?

 

[Chickntrk]

EGR has been unhooked since 2018

 

[warrpath4x4]

where it happens when its cold, i wonder if the coolant temp sensor might be going out. Its a simple and cheap thing to replace. Ive had two 89's that were running funny and that was the issue as it feeds the ECM.

 

[Chickntrk]

I was wondering that myself, because it seems to disappear once it’s warmed up

 

[gmbellew]

I was wondering that myself, because it seems to disappear once it’s warmed up

the only difference to the ECM when "warmed up" is it goes into closed loop and starts using O2 sensor feedback to adjust fuel delivery. I'd think if the CTS were on the fritz, it would also affect cold starts and drivability when in open loop.

 

[Chickntrk]

The problem mostly happens in open loop though, of course you’d think if the CTS was faulty it would cause issues in closed loop to

 

[gmbellew]

could be an intermittent vacuum leak that goes away when the engine is hot? or maybe a sensor is on the fritz, but acts normal when it gets hot bc things expand and the connection improves?

intermittent issues are tough. being able to watch the data stream, even though it is a slow stream, would help diagnose it.

 

[Chickntrk]

Here’s the plugs, 2 from each bank. This morning it was about 30 degrees here. I started the truck and let the windshield defrost (10-15 mins idle) and when I pulled out all was well so it definitely only does it when it hasn’t warmed up. Outside temp doesn’t seem to matter tho it does it whether it’s 80 out or whether it’s 30 if the coolant isn’t up to temp.