Power problems after top end rebuild

[clandr1]

Hi everyone, I hope you can help with a problem that's just recently come about.​

I had everything rebuilt/replaced from my head up (pistons & rings looked good so block stayed as is) and after the rebuild I was getting a check engine light for codes relating to O2 sensor and TPS (throttle position sensor) voltage. I replaced the O2 sensor and reset the codes, and since then I haven't had the check engine light come on.​

However, I have been experiencing intermittent idling issues (surging idle sometimes) and the motor seems to "bog down" on occasion when I put the accelerator to the floor.​

Do you think this is TPS related? Should I bite the bullet and buy a new one and see if that fixes the problem? FYI it's a stock '89 with factory TBI 350.​

Thanks in advance for any help!

edited to add solution found!
​

The timing had been set with the EST wire CONNECTED, which was causing my idle to be off. I had him reset the timing with the EST wire DISCONNECTED and it has been running great for 3 years.

 

[Fordum]

Bogging down when you try to accelerate, is a classic TPS symptom. BUT, they are easy to check if you have an old needle type ohmmeter before you replace it.

If the surging idle and bogging started as soon as you got it back together, odds are its a vacuum leak.
Probably the intake gasket.

There are several ways to check for this. A smoke test is best, but you need a smoke machine, or be a heavy cigarette smoker..........

Another quick test involves a propane torch.
Let the engine idle, turn on a torch but don't light it. Put it next to the intake gasket, and if there is a vacuum leak, the engine will speed up when it sucks in the propane.

Other folks spray carb cleaner or WD-40 around the gasket area and watch for the engine speed to change.

 

[clandr1]

Thanks for the info. What would the solution to an intake gasket leak be? Torque the bolts tighter?

 

[Fordum]

If they were torqued correctly the first time, it won't help. If not, the intake needs to be taken back off and torqued correctly.

Most likely something shifted with the gasket when it was put on. I seem to remember something tricky about the middle at the back.
But, you need someone who has rebuilt more of those engines than I have to tell you for sure.

 

[Richcz28]

Throw a vacuum gauge on it is the easiest way to test if you have good vacuum. It can differ depending on cam size and other variables, but most chevys I've had a gauge on were between 12 and 17 at 700 rpm idle. After that do the leak test as stated above. You can run through the torque pattern on the intake again if it seems like there is an issue. Otherwise your gasket size / mating surface could be at odd angles to each other.

 

[clandr1]

Where/how do you attach a vacuum gauge?

 

[Richcz28]

Any vacuum line that fits on the gauge that runs to an accessory you don't necessarily need or throw a t in.

 

[Richcz28]

I had bad vacuum one time because of a leaky egr diaphragm. You can also try blocking off accessories that run off vacuum to see if it changes on the gauge. Lots of factors come into Play after that much work. Old line that are stiff can lose vacuum too.

There is also the rag over the carb / tbi test. If it doesn't stall right away or runs better your engine is getting air elsewhere.

 

[4X4HIGH]

Lets first find out if you timed the engine correctly. You MUST disconnect the EST (Electronic Spark Timing) wire BEFORE setting the timing to ZERO. The EST wire is a single brown wire located near the brake booster.

 

[clandr1]

Quick update guys -- I actually paid a guy to do the work, he checked it out a couple weeks ago and couldn't find any vaccuum leaks (said he used a gauge) and tested the TPS and said it was working fine. He thought it might be clogged injectors, but I didn't have this problem prior to the rebuild. 4x4HIGH, he said he checked the timing, but is it possible he could've checked it without disconnecting the EST wire first? I'll have to see if he did or not.

Thanks again for all the suggestions.

 

[4X4HIGH]

If the person is not familiar with GM vehicles then he could have easily put a timing light on it without disconnecting the EST wire and it would have read about 20* advanced.