I have a 96 LT4 installed in my 69 camaro. It had been starting fine until this weekend. The only thing I had done was install the throttle cable and hooked up the vacuum line to the brake booster. I lowered it off the jacks and it would crank but not start.

The engine has an OBD1 engine harness and programmed computer on it (OBD1 knock sensor). At Key on, the check engine light never turns off. When I scan for codes using Free Scan. No codes are found. I pulled the MAF connector - no codes. Pulled the TPS - no codes. Pulled the MAP and got a code and saw the MAP pressure on Free Scan change.

Yesterday, cranking the engine, I did get a 41 to trip. Diagnostics led me to replace the ICM. I replaced the ICM and the code has not returned. I ohm'ed the check engine light back to the PCM connecter and got 0.3 ohms.

I believe I have a true fault though it seems Free Scan is not picking it up. Any suggestions on other checks or should I look for some other software to scan for codes?

Thanks,
Kevin
69 camaro LT4 w/hot cam T56 9" 3:90 Baer brakes mini tubbed Budnik's DSE suspension components Vintage Air

 

I have read conflicting information on the check engine light being on at "key on engine off". I thought it came on for a couple of seconds then turn off. Correct?
Thanks,
Kevin

 

Just checked this on my '94 Z28 .. With key on / engine off the Service Engine Soon (SES on dash, called MIL in documentation) and the Check Gauges (turned on by low oil press or low alternator voltage) indicators stay on. As far as another software... Download TTS Datamaster EE (F-Body '94-95 OBD1) software and you can get 20 free logs. I have never heard of FreeScan not displaying any DTC's. As far as your test.... TPS codes are DTC 21 and 22, a 21 will only happen w/engine running and a 22 (low sensor voltage) will not occur if connector is pulled at the TB. MAP codes 33 and 34 both require engine running to set so no DTC after pulling connectors could be normal. To test a DTC pull the fan relay or fuse it will throw a DTC 77.

 

Bob, thanks for the info..
I did download datamaster on Sunday. It did not show any DTC's. The engine did fire on one cylinder for a second and the check engine light went out.

Am I on the road to an opti replacement? White wire from PCM to ICM has 2.5 volts on AC. Battery voltage on the other two wires and ground is present on the other wire. Coil measured 5800 ohms. Disconnected the MAF, still didn't start. Newer wires and they are all seated. Is it worth checking spark? Pulled one plug, there is some fuel on it.

Thanks,
Kevin

 

To clearify. you have a '96 LT4 engine with a '94 OBDI PCM in a '69 Camaro. What year engine harness are you using ? What year coil and ICM are you using and I assume you left the '96 opti in there. here is a schematic of the '96 ignition ref shoebox http://shbox.com/1/1996_ec_03_ign_system.jpg verses the '94 setup here http://shbox.com/1/95_ign_system_schematic.jpg they are slightly different. Sounds like you already did this checkout http://shbox.com/1/4th_gen_tech2.html#opti_test . The 2.5V AC on the white wire indicates the PCM is switching the +5V/0V signal to the ICM, so you s/b getting a good clean hot spark at all plugs. I would check for spark to eliminate the wires, coil or the HV section (cap/rotor) of the opti. I assume you verified plug wires are not crossed and are correct at the opti end. Was the engine opened up (cam install) or the opti removed during the swap (verify correct opti installation) http://shbox.com/1/opti_back.jpg. Your coil resistance is a little low (5.8K Vs 8.5K), but I don't know if it's enough to cause spark problems, maybe someone else may have more experience with this !! or you could get it tested .

 

Bob,
Yep, it's a 69 camaro with a 96 LT4 with hot cam and 30# inj. 94/95 OBD1 PCM (PCMFORLESS program), aftermarket harness for 96 engine. Running MAF. It's facing the correct way for air flow. Wiring from ICM to coil matchs 96-97 schematic. Running 96 ICM, coil, and opti. Replaced ICM and coil within the last couple of days. Have OBD1 knock sensor
History: The engine sat for about 10 years. Installed new 30 lb injectors and got engine running. While the car was out getting painted. I installed the hotcam (really considered putting new opti in at that point). Re-installed engine when car came back. Had PCM calibrated for hotcam and 30# injectors. I had been starting the car every once in while. Lowered the car on the ground to see if it would move on it's own in over 10 years. Well, here we are. The only thing I did between it starting and not is installed vacuum line to boost which I removed and put a plug in the intake and attached the throttle cable.
If the car sits over night, it will hit on a cylinder on the first crank then it's all over.
Fuel-Air-Spark. Have fuel pressure. How much fuel? Air - Assuem the air metered by the IAC during crank? Spark - finding out.
I'll check the ignition wire, ect.

Thanks for the help.
Kevin

 

Little confused with this statement, did you mean vacuum line brake booster as in your OP. If so could this have created an intake air leak if something is wrong with the brake booster. Also what is your TPS voltage with TB closed (TPS=0%) ? It s/b between .55 and .65 volts.
The IAC during cranking is controlled by a PCM table called "IAC Park Position Vs Coolant Temp" that varies air input by coolant temp, and the fueling is controlled by a combination of 6 (3 Crank AFR, & 3 Prime Pulse width tables) a fairly complex setup, usually never needs altering unless engine has very large injectors added. The AFR is all PCM guesswork since O2's are not function at that point and large injectors throw it off.

 

Sorry, yes the vacuum line going to the brake booster. After it didn't start, that was the first thing I thought of. I have put a plug in the intake to block the port.
I do have spark at the plugs. It seems to spark brighter on the first firing than the others. It acted like it wanted to started. I removed the fuel pump relay. Probably should have removed the injector fuse. Will look at it again tomorrow night. Also charging the battery. It was down to 10 - 10.5 volts cranking.
TSP showed 0% voltage was at 0.69v. IAT and ECT were at ambient temps ~13C. MAP 98.5 kPa. Spark advance at 13 degrees during crank. Ohmed the IAC at the PCM connector got 53 ohms between "A" to "B" and "C" to "D". Ignition wires are routed correctly

Thanks,
Kevin

 

Here's an update. Installed a new GM opti-spark. The spark at the plugs wasn't as bright as coming out of the coil. Same concern. Verified the wires were routed correctly.
I can get the engine to start if I hold the throttle full open. It will idle once started.
Any ideas?

Thanks,
Kevin