I have an 89 Trans am with a 305 TBI. I just rebuilt the motor (less than 200 miles on it) and recently the car won't hold an idle to save its life. I washed the car one day, and took it out for a test drive afterwards when the car would stammer and shake whenever it the RPMs dropped below 1000 to the point where it would barely run. When it is stammering, I get a hell of a long delay trying to rev it up, but once it rises past 1000 RPM the roughness is hardly noticable and the car revs fine.

I'm getting codes 23 (low IAT reading, assuming this is because I keep unplugging the sensor when I remove the air cleaner) and code 42 (replaced the ESC twice already).

So far I have checked the fuel pressure (constant 11psi), replaced the fuel pressure regulator, replaced the ESC module (with one out of my TPI monte carlo) checked the timing (4* advance and the timing mark does not move an inch when the esc is disconnected no matter how crappy the idle is, it advances smoothly with the esc connected), replaced the cap, rotor, and plugs (all the plugs were fouled badly), checked for any possible vaccuum leaks, checked the TPS (.54 at idle, 4.4 at WOT) and cleaned the IAC. Nothing I've done has made one bit of difference.

After the rebuild the car ran beautifully (once I replaced the faulty ESC module, computer, and ignition module (missed a few grounds putting the motor back in, oops)) but after washing it once, the car now will barely run.

Anyone have any idea where I should go from here? I am seriously running out of ideas. I suppose next I'll do a compression test to see if I screwed up anything on the top end of the motor, but I doubt this is an internal motor problem. I'm hoping I'm missing something stupid and this will be an easy fix, anyone have a clue?

 

if this car has a map sensor, i would check that. I did the same thing, and found a vac. leak. The map sensor(if equipped) would send a faulty reading and the computer would not adjust right.


Good luck.

 

Replaced the MAP sensor (junkyard), and the car seemed to run very well for a little bit.. as soon as i revved it however it immediately stalled out... Checked the hose running to it and the connector, etc and reset the computer and now the car runs exactally the same as before. I checked the timing again and with the ESC connected the timing mark seems to jump a bit around 2 to 8 degrees advance but mostly sticks around 4. I tried running the car with the MAP unplugged and it seems to run a little better, but there is still a long hesitation revving it up from idle and it is still running nowhere close to smooth...

Anyone else have an idea? I'm about to just give up and take it to a shop and get charged for them replacing every sensor on the car...

I'm resetting the computer again now and I will take it out on a drive to see if I get any new codes... Is it possible that I'm somehow frying MAP sensors?

 

Car stumbles and stutters and still wont idle smoothly at all... Computer isn't throwing out any codes... Double and triple checked all vaccuum lines and still nothing. I think tomorrow I'll try a new ignition module (since the one on there now has a small crack where the plugs connect to it that may have gotten wet) and see if that makes a difference... Did I stump everyone?

 

just throwing out some more ideas (wouldn't say they are good, but they are ideas. And they'll keep you busy for a while longer )

did you check compression yet?

some coolant leak? do a cooling system pressure test.

How do the plugs look now after you replaced them?

how do cars behave when cat clugs up?

How did you check for vaccum leaks, break cleaner? What if some other component that uses vaccum went bad, like the brake booster? Try clamping down the vaccum lines one by one. Then again, vaccum leak is usually higher than normal idle, right?

Already checked the timing with vaccum advance unplugged, right?

didn't think EFI system would have fuel pressure at 11psi. I thought they were close to 40-45psi, but maybe that's TPI only. I am very far from expert on this though.

 

Thanks for the ideas... havent checked compression yet but I don't think thats the culprit anymore. As for coolant, there is a small leak around the intake where the gooseneck for the thermostat goes but thats the least of my worries... Haven't pulled any of the new plugs yet, but I'm just going to assume they're as fouled as the old ones already.

I've thought about the cat being clogged, but I'm still getting strong exhaust coming out of the tailpipes and the odds of the cat getting clogged right at the point when I cleaned my car are slim to none...

I know its not a simple vaccuum leak... I've unplugged several vaccuum lines while the car was running and the idle surged a bit and that was it.. There is still a strong vaccuum going to the MAP sensor.

The timing is computer controlled and I've checked it with both the ESC unplugged and with it plugged in.. both behaved as they should. As for the fuel pressure, TBI should be running between 9 and 13 psi, so 11 is right on the money if I recall correctly. Thanks for the help though... I'll see if I can get to a compression test tomorrow...

 

I've seen friend's 73 vette that had internal coolant leak. You couldn't even see it from the outside, but the seal between the intake and the head( I believe that's what he said after mechanic was done with it), was leaking coolant into the intake ports.

good luck

 

No sweet smell or white smoke coming from the exhaust... All I get is the strong gas and exhaust smell from the car running rich as hell..

Also pulled out the injectors and took a look at them and cleaned them a little bit.. spray pattern looks better now. Checked the timing again... nothing... Pulled the IAC again and cleaned it with the pintle out this time... Now the car wont even idle... It will start up when I give it a little gas and will rev up fine... let off of the gas though, and it just dies... I need to just burn this damn thing to the ground..

 

Replaced the ignition module again, checked the voltage coming out of the MAP sensor.... voltage seems right on, car still runs like complete and utter ****....

 

I know I'm pretty much talking to myself now, but I've switched out the distributor with an extra (out of my TPI monte thats half done), and replaced the IAC and CTS with brand new ones.... Once again, absolutely no change... I think I may try a new MAP sensor next, as that was the only sensor to make a difference. I'm also thinking about trying data logging just to see if I can find the problem... Any ideas or recommendations would be appreciated...

 

And the problem has been found...

Compression test:

Coming in first, cylinders 2 and 6 with 150 psi
Second, cylinder 7 with 140 psi
A three way tie for third, cylinders 1 3 and 5 with 130 psi...

And bringing up the rear, cylinders 8 and 4 with 5 and 2 psi respectively.

Time to tear apart the motor again....

 

Jesus christ.... Bent valves?

 

Not that lucky, I get 120 psi when I squirt oil in the cylinders... Hopefully a re-ring will do it, but with my luck, it seems I'm out one newly rebuilt motor. Guess sometimes rings don't want to seat right, or maybe I took it too hard on her after the build...

 

With those compression numbers, it's hard to believe there are even rings at ALL in there....

Don't feel too bad, my 302 crate motor lasted 10,000 miles and blew. 85 psi compression on the dead cylinder, and 95% leakdown.... Turns out the bore is gauged, all the way around, very deeply.....still don't know why, it makes no sense.

And the two cylinders adjacent to it both had scratches/gauges in the bores, one so deep that it may take .040 to .050 over to remove it. Basically my brand new $1200 shortblock is a large paperweight.

I just picked up a junkyard block and am cleaning it up....it's in much better shape, aside from the crappy cast pistons.

 

Yeah, with the amount of fine metal shavings I found in the oil, I'm sure there isnt much of a ring left... Maybe I'll have another blown up piston to add to my collection (now consisting of a piston out of my 2.8L 84 ranger with the pin breaking it in two and the rod bent almost 90*... Fun siezing a motor doing 70 with a manual tranny... sheared off all of the flywheel bolts too). For some reason my father keeps telling me that I can just pull the motor and throw in a new set of rings... I'd love to see how scored up these cylinders are... Thanks for the encouraging story of destruction.