92 LSC idle problems

[LSC_dude]

I just recently swapped motors in my LSC from my "90 donor w/ 38,000 miles. With everything completed, the car now has some serious issues trying to stay running. The rev's fluctuate abot 800 rpm for a while and than dies. I have searched all over for any vac leaks but all seems ok there. The timing is good and both engines ran great before the swap. Any suggestions?

[gadget73]

How many inches of vacuum do you have at idle? Have you checked for codes? It sounds like classic vacuum leak problem, or possibly a temperature sensor issue.

[NYC LS8]

Put your old IAC on the "new" motor. Or clean/replace the one that's on it now.

[Hizhonor]

I just recently swapped motors in my LSC from my "90 donor w/ 38,000 miles. With everything completed, the car now has some serious issues trying to stay running. The rev's fluctuate abot 800 rpm for a while and than dies. I have searched all over for any vac leaks but all seems ok there. The timing is good and both engines ran great before the swap. Any suggestions?

Did you swap out the computers, too? (ECM) Did you check the hose to the MAP sensor? I agree with Tommy ... check the IAC, also. How about the plug gaps and plug wires? One crossed wire or a couple og plug gaps off will wreak havoc on the engine too under load.

[marked for life]

Start the car and have some one hold the rpm's up a bit to keep it running (steady) then spray starting fluid all over in the intake area and listen for the rpm's to pick up, also check throttle posistion sensor voltage by back probing at the connector to make sure it is under 1 volt at (trying) to idle speed codes can tell you this, As said are there any codes ? Was the intake off ?

[LSC_dude]

Did you swap out the computers, too? (ECM) Did you check the hose to the MAP sensor? I agree with Tommy ... check the IAC, also. How about the plug gaps and plug wires? One crossed wire or a couple og plug gaps off will wreak havoc on the engine too under load.

I did not swap the computers as I was informed all was the same. The hose to the MAP was replaced when I removed the intake to change gaskets + valve covers. Plug gaps are all good and new wires have been put on it along with Cap/Rotor and coil. IAC has been removed and cleaned. I will be checking vacuum tomorrow and looking at the TPS again.

[LSC_dude]

Start the car and have some one hold the rpm's up a bit to keep it running (steady) then spray starting fluid all over in the intake area and listen for the rpm's to pick up, also check throttle posistion sensor voltage by back probing at the connector to make sure it is under 1 volt at (trying) to idle speed codes can tell you this, As said are there any codes ? Was the intake off ?


No fault codes have been given. I'm gonna " borrow " the MAP off my buddies 'stang and see if thats the prob. Will keep you posted

** Thanks for all the replies... Much appreciated **:)

[JBLSC]

air charge sensor and temp sensor not calibrated to eachother would cause crapy idle

[LSC_dude]

air charge sensor and temp sensor not calibrated to eachother would cause crapy idle

How does one calibrate the air charge sensor to the temp sensor? All other attempts to correct have failed. I am one step closer to taking off the intake and starting from scratch. :confused:

[psychotexan]

i believe the computers are different between the lsc and the bb.i may be wrong

[psychotexan]

also could it be surching for idle?mine did that this morning cuz the battery was dead i need a new battery.

[markedman90]

Start the car and have some one hold the rpm's up a bit to keep it running (steady) then spray starting fluid all over in the intake area and listen for the rpm's to pick up, also check throttle posistion sensor voltage by back probing at the connector to make sure it is under 1 volt at (trying) to idle speed codes can tell you this, As said are there any codes ? Was the intake off ?PLEASE don't use starting fluid !! USE WATER and see if it stumbles. I went thru this w/my '90 having the intake(s) off & on a couple of times.
If there's an external (to the atmosphere) vacuum leak, the engine will stumble when water is sprayed around the intake. It could also be sucking coolant. Mine did it at 3/4 and was very close to hydraulicing #3 before I found the problem.
Used intake hardware & corrosion = leaks of one sort or another.