89 GM throttle body low vaccum on front ports of Throttle body

I have an 89 gmc sierra I just put a motor in and am expericening idling problems and acceleration problems. It acts like i'm losing vaccum somewhere but have checked several times and found no leaks. The front 3 vaccum ports of the throttle body have very little vaccum while the rear port that runs to the map sensor has normal vaccum...I have replaced the throttle body gasket and thoroughly cleaned the throttle body...does anyone have and ideas. I've been working on this truck for months and its getting very frustrating


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have you sprayed carb cleaner around the intake gaskets to check for leaks? If you find a leak it will rev the engine momentarily.

yes i've sprayed all round the base of the throttle body and around the intake. and i've also checked with the hose trick, i'm not getting any engine trouble codes. It acts just like a vaccum leak though i just can't find it

Is it backfiring? If so is it out the intake or exhaust? Have you checked the egr valve?

no its not backfiring and the timing is set dead on. i checked and cleaned the egr a while back but i think i will take it off again tomorrow, something might have happened during the engine swap but it would have to be stuck open becuase there isn't enough vaccum coming off the throttle body for the computer to open it correctly

Get a vacuum gauge and see how much vacuum you have. What cam is in it?

Ive seen the the ports right full of crud before. Pull the TB off and have a look at the bottom side of it.

Ron has my thoughts exactly. You mentioned you cleaned the TBI but did you run something through each port to assure it was clean? TBI engines have a real bad habit about carbon build up. When I removed the one off my truck I found one completely caked shut with carbon and the other two were almost just as bad. If it's getting normal vacuum at the rear port then it should be on the front ports which points to there's got to be some blockage somewhere.

thanks for the suggestion guys, i actually did clean those ports a few months back when this throttle body was running fine on another motor but i will pull it off and check again. I didn't think to clean them again as it was running fine on another engine just a short time ago but perhaps they've become clogged up again...I will let you guys know what i find

thanks guys, that seems to have taken care of the vaccum problem, now on to the next issue..i'll be driving it again before i know it lol