350 tbi stumbles and backfires under load

[lctopal]

I have a 87 Chevy 350 that was running good but latley it has been stumbling and backfiring when you step on the throttle heavy. It revs up fine in park and low acceleration but doesn’t like to accelerate when you step into it a bit

I’ve change plugs, distributor rotors, distributor cap, esc module, map sensor and did the plug wires about a year and a half ago , having no luck figuring it out
What should I be checking

 

[Ricko1966]

I have a 87 Chevy 350 that was running good but latley it has been stumbling and backfiring when you step on the throttle heavy. It revs up fine in park and low acceleration but doesn’t like to accelerate when you step into it a bit

I’ve change plugs, distributor rotors, distributor cap, esc module, map sensor and did the plug wires about a year and a half ago , having no luck figuring it out
What should I be checking

Check compression and vacuum readings,first.

 

[PrairieDrifter]

Sounds like too much timing to me.

 

[F-64]

I had to buy the old snap on red brick scanner MT2500 to finally diagnose my 350 tbi. You'll need the proper catridge, aldl adapter and power cord. Mine was close to $300 total, but you can find them for a lot less.

It does help to have a scanner if possible. My O2 sensor was being thrown off by bad valve seals.

You should do the vacuum, static and dynamic compression test. You could also block off half of your intake with tape to see if the situation improves. Then remove the blockage and create a small vacuum leak to see if that helps the situation.

Check your timing using the proper method shown on your emissions sticker under your hood.

 

[Radiohead]

Is the accelerator pump squirting?

 

[Redfish]

Is the accelerator pump squirting?

TBI. No accelerator pump.

 

[Radiohead]

Doh! Missed that.

While still on the fuel idea... Bad fuel delivery, like pluggy filter ot weak pump?

 

[lctopal]

Doh! Missed that.

While still on the fuel idea... Bad fuel delivery, like pluggy filter ot weak pump?

I have changed the fuel filter I’m going to check the fuel pressure tonight and the little screen filters on the injectors

 

[lctopal]

Sounds like too much timing to me.

I have double checked the timing at it looks to be good

 

[lctopal]

I had to buy the old snap on red brick scanner MT2500 to finally diagnose my 350 tbi. You'll need the proper catridge, aldl adapter and power cord. Mine was close to $300 total, but you can find them for a lot less.

It does help to have a scanner if possible. My O2 sensor was being thrown off by bad valve seals.

You should do the vacuum, static and dynamic compression test. You could also block off half of your intake with tape to see if the situation improves. Then remove the blockage and create a small vacuum leak to see if that helps the situation.

Check your timing using the proper method shown on your emissions sticker under your hood.

I will definitely be looking for a code reader, I will have to take a look at the emissions sticker and ensure I set the timing correct, I will look further into the vacume system next time I get a chance to work on it

 

[gmbellew]

.

 

[fast 99]

Lean, low fuel pressure think the specs are 9-13 they will not run near the lower spec, fuel volume plugged filter, have seen defective off brand distributor caps crossfire buy a Delphi they are expensive, bad coil, corroded coil tower, bad plug wires. Check vacuum if uneven suspect valve train issue ie: broken spring, flat cam, ect.

 

[PrairieDrifter]

If timing is good you have most likely a fuel issue. Could be ignition or a vacuum leak too

 

[Ricko1966]

If you have uneven compression,a valvetrain issue or a vacuum leak, you will never get TBI to run right. It is manifold pressure controlled. That is why I said run vacuum and run compression, if the manifold absolute pressure sensor (MAP) isn't getting the readings it needs to see, the ecm can't regulate timing and fuel correctly. And as others have mentioned check fuel pressure. There's of lots of things that can cause the problem, start with basic diagnostics first.

 

[lctopal]

If you have uneven compression,a valvetrain issue or a vacuum leak, you will never get TBI to run right. It is manifold pressure controlled. That is why I said run vacuum and run compression, if the manifold absolute pressure sensor (MAP) isn't getting the readings it needs to see, the ecm can't regulate timing and fuel correctly. And as others have mentioned check fuel pressure. There's of lots of things that can cause the problem, start with basic diagnostics first.

I have checked the fuel pressure and it’s right around 11-12psi so that should be good I didn’t see any drop in pressure with reving, I pulled the vacume line off of the Val cover and it has good suction when I put my finger over it, I don’t have the proper tooling to check the acual vacume but I will track it down and check, one thing I noted when I covers the line to the valve cover the idle does come down a bit