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Old 03-02-2011, 07:51 AM
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I need Help with my 305 TBI please

I Have a 1991 GMC sierra short bed reg cab with a 305 tbi in it. I recently had to change the intake manifold, and exhaust monifolds. I changed the manifolds to 1 5/8 hooker shorty headers.

When I let my truck warm up, and idle it out the driveway it runs very rough, like it had a lumpy cam in it. Drives fine down the road untill you get into town where I have to drive slower, and it runs the same way. (Like it has a lumpy cam in it). I really need some help with this, this is my daily driver and it is embarassing to drive it through town like this.

Any help on this matter would be greatly appreciated

Thanks

Ed Williamson

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Old 03-02-2011, 08:16 AM
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vacuum leak or the MAP is disconnected/ malfunctioning
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Old 03-02-2011, 08:34 AM
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Quote:
Originally Posted by 400smb_s10
I Have a 1991 GMC sierra short bed reg cab with a 305 tbi in it. I recently had to change the intake manifold, and exhaust monifolds. I changed the manifolds to 1 5/8 hooker shorty headers.

When I let my truck warm up, and idle it out the driveway it runs very rough, like it had a lumpy cam in it. Drives fine down the road untill you get into town where I have to drive slower, and it runs the same way. (Like it has a lumpy cam in it). I really need some help with this, this is my daily driver and it is embarassing to drive it through town like this.

Any help on this matter would be greatly appreciated

Thanks

Ed Williamson
Did you use the correct procedure to set the timing after removing the distributor? Is the check engine light lit? Have you pulled the codes that may be stored? That info would be the first thing I'd want to look at.

If the battery was disconnected during the work you did, the PCM will need to "relearn" during the next few days. It can take a while for this to settle down and run right, so if you just started driving it again after the repairs, I would keep driving it unless there are stored codes.

This all assumes there are no vacuum leaks (the intake gaskets are especially prone to leaking) and the O2 sensor is functioning, etc. The codes that may be stored will tell you if it and the other sensors are OK, so be sure to read the codes.

You said you "had" to replace the intake, etc. Was it cracked?
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Old 03-02-2011, 08:45 AM
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It wasnt crack at first. The Heater hose that runs into the back of the intake near the dizzy rotted off. So I took a easy out to get out the rest of the pipe and I broke the whole housing right off it. How do you know if you have a vaccum leak through the intake. I replaced the gasket and at the front and rear I put high temp gasket silicone in each corner where the gaskeets meet and put a thin line below and on top of the rubber gasket.

I just put a new o2 in it 2 days ago and that was the only code that was stored on the pcm
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Old 03-02-2011, 09:06 AM
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If there's air entering w/o passing through the throttle body, it will cause the PCM to 'think' the mixture is lean. This will cause it to enrichen the air/fuel mixture and this can cause drivability problems.

It may be hard to tell if there's a vacuum leak if it's small. You can use a vacuum gauge to see what the reading is, but w/o knowing what it was before the repairs, it will be harder to tell. Instead of me relaying everything to you about this, there's a pretty detailed article on all this HERE.

HERE is a vacuum gauge reading guide, of sorts.

THIS is another one.

You might want to go back and retorque the bolts. By now, the exhaust bolts will likely need tightening and checking the intake bolts would be a good idea as well.

How did you reset the timing? There's a blk/tan wire that should be disconnected before setting it to TDC. Was this done?

It may just need more time. How long have you driven it since the repairs?
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Old 03-02-2011, 01:04 PM
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I found the problem i had the wrong vaccuum line one the map sensor
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