If you have a serious vacuum leak it also could be an intake manifold leak.

 

has the car ever run right?

i havent seen your datamaster or .csv file but what id check is the IAC very closely. hook up a scanner and open the tb blade stop screw like mentioned before to allow some of the air to bypass the IAC and see if that lowers the value any.

its hard to pick out what could be causing what though since you have so many odd readings.. MAP, spark timing, IAC. if the idle is really low like in the 500 rpm range then the computer will call for the IAC counts to increase in order to raise the idle back up to the target RPM. have you tried starting the car with no air filter on it?

 

Ok, i'll offer my last possibility....jumped timing. With that many miles on it, 178k, it could have happened. On distributor equipped SBC's we could watch the rotor in the distributor while turning the crank balancer bolt back and forth to check for slop. The two movements should track each other relatively speaking. The crank turns twice for each camshaft revolution. Best done with the spark plugs out, though. On a LT1, of course we cant see whats going on in the opti.
Doug

 

Yes, usually it's w/o air filter, but it makes no diff. either way....2-nite I'm going to try to adjust the Idle screw.


And according to the previous owner, the car ran "flawlessly" before this happened....Which, again, is why I've been straying away from mechanical problems, they usually dont happen "one morning when he woke up"......

 

Here's the latest round of tests.

I re-checked the calibration of the MAP sensor:

IN Hg Datamaster Voltmeter
0 29.54 4.763
5 23.54 3.64
10 18.20 2.72
15 13.02 1.79
20 8.40 .950
25 3.17 .036


I also rang out the wires from the PCM to the IAC, they rang out ok.

I put a screwdriver in front of the throttle adjustment screw, and it took the tps signal from .62 to .82, and the IAC did respond! It throttled down to 18 counts. The idle did change after the IAC throttled. Then I tried to rev it up, and it died.

I tried wiggleing the spark plug & coil wires around while idling..Nothing

I do believe that the IAC is trying to compensate for the rich mixture.

I re-checked all the ECM grounds and they all read <.3 ohms

Are their other checks I can do before I tear into it mechanically?

 

Shoebox, I also re-hooked up a vacuum source to the MAP sensor, and varied the vacuum. This time I had the datamaster hooked up....I can e-mail you the file if you like....Same thing happened, the car died at anything other than actual vacuum....

 

it sounds like you have plugged cats

even if you dont running rich enough to cause glowing red manifolds will screw up your cats.

drop the y pipe and fire it!!!!!!!!!

it sounds like youve tried everything else

 

I just found an interesting test in the service manual...They've got a test for restricted exhaust system!!!

You remove the AIR system check valves, each one right after the connection tube on the exhaust manifold, and plug in a pressure guage with the correct rubber fitting connected (large as you can get that seals the tube). I could only get to one side, but it says at adle back pressure should be <1.2 psi. Mine was at 1.0.
At 2000 rpm, backpressure should be <3.0 and it's ~2.0.

Is this worth dropping the cats?

What do you all think????

 

so you are saying you have less backpressure than you should?

 

Man...it sure sounds like a timing issue to me.....I would double check the chain setup and make sure the opti isnt off. Make sure when you set everything up that #1 is at TDC on the compression stroke.

 

Well im no expert but this is what i think, if the exaust isnt getting through the cats than its backing up all the way to the manifolds which is getting them hotter than normal which causes the glow. Is the car poping out the exaust pipes? That would be another symptom. Local z's around here were losing there cats round 100k-120k miles. It takes all of 20 min to gut a cat so 40 min an your set since u have 2. Its an easy try and if it doesnt fix it well then you got 10 extra horses for when you do get it right. good luck.

Zach

 

Thinking logically about this though, if he has less backpressure than the manual says he should have, then he does not have plugged cats, or else he would have much more backpressure than he does. Rule out plugged cats in my opinino.

 

Agree.



You are sure the valves are not too tight?

Pick an easy cylinder to get to and do a compression test. If valves are not too tight, maybe you do have a cam timing issue.

 

man i hate to say this, but maybe one of the parts you bought its faulty, my icm went out and after a month of puzzles it came out to be that the one i bought from napa was a bad one, just my 0.2

 

I mentioned the cam timing yesterday, if you recall. The reason is that you continue to suspect all manner of sensors, all of which will trip a code. The idea that none of them do, can only point to a PCM failure, which is unlikely. What it is, is something other than a PCM monitored function.
Maybe I am wrong. But it is simple to check for cam timing by placing a piston at TDC on exhaust and observing that the overlap period is split equally on either side of TDC. It is still worth checking, and ruling this out as a possiblity.
The check for exhaust backpressure is also easy. Simply use a large rag on the tailpipe, and rev the thing up. If you don't get a large blast of fast moving exhaust gases, the convertor is plugged. Takes 15 seconds.
All that the red manifolds are telling you is that combustion is occuring in the exhaust stream, rather than in the cylinder. Whether it is cam timing, lean, rich, or whatever, is not the point. If the O2's are crossing over the .5 millivolt line quickly and regularly, then the mixture is so close it doesn't matter.
Check the cam timing. Rule it out, or find out that it is the problem.