Hey guys. I recently bought a z28 with a starting problem, some of you may have seen a few posts about it already. The car fires up and runs perfectly about 10% of the time, for the other 90 it is hard to start, requires a bit of throttle to fire and keep it running and once its running, it runs very rough for about 20 seconds, it doesn't missfire but it seems like its only running on a few cyclinders as it shakes pretty bad. After about 20 seconds it seems to "clear out" and you can take your foot off the pedal and it runs perfect. My mechanic hooked it up to the scanner and after some testing told me it was a bad ECM, the ecm was sent away and rebuilt, I re-installed it, and the car started and ran like a charm, I was happy the problem was fixed, until I shut it off and tried to start it again only to find the same problem. I'm getting frustrated now obviously, the mechanic says the next thing he would try is the security system bypass kit and then the optispark. If I had a spark problem wouldn't it run rough all the time? And an ses problem would make it die after start up correct? I'm out of ideas where I should start next and need some help, thanks in advance

 

Should have mentioned car is 94 lt1 automatic

 

Check the temp sensor wiring, that's been known to cause misfire. While your at it, check all wiring and connections under the hood.

 

Glad to hear that answer, I was just going to do a check of all my wiring, which temp sensor are you talking about?

 

Don't bother with the security bypass. VATS keeps the engine from cranking, but your starter is able to spin the engine, so VATS is OK.
The temp sensor is the one for the PCM, located on the water pump. If you have a scanner, compare the reading on the temp gage with what the PCM sees, they should be close.
http://shbox.com/1/temp_sensor1.jpg
If the engine is running rough and on just a few cylinders it is misfiring. Misfire is like pregnancy, there is no such thing as a partial misfire.

A lit SES does not make an engine stall, the SES is just a trouble indicator.

Get the thing scanned and check the fuel pressure with a FI gage. Hard starting is often caused by the FI system loosing pressure when the engine is off. Try cycling the key On & OFF several times to prime the system, before attempting to start ans see if it starts right up.

 

Ive had a fuel pressure gauge on the car and everything checks out. The car actually starts better if I do not prime it, taking the key from the off position directly to the start position, without priming. If I prime it more than once it just cranks and will not fire until I let the fuel run down for a few seconds. Unfortunatly I do not have a scanner. I had it in for a safety and to have the starting problem addressed, and the mechanic said that while he had his scanner or computer (I'm very unfamiliar with the electronics of cars) hooked up to the car, It would not act up for him. The car has no engine lights on, no security lights, nothing, and like I said after about 20 seconds, the engine seems to clear and all is well, I drove it 60kms with no troubles whatsoever, WOT causes no hiccups or anythings, it runs 100%

 

So with all the pieces of the puzzle, it looks to me like its more a mechanical type problem rather than an electrical one. Something might be wrong with the fuel pump. Isnt there a one way check ball or valve in the pump? Maybe also something wrong with the return portion of the fuel system. Fuel pressure regulator?. maybe theres something in the tank itself.... maybe a shop rag floating around in there??? Im just throwing things out there.... but I have read the shop rag story more than once...... also left in oil sumps as well. Keep driving and using the car, the problem will get worse and soon make itself evident.

 

By "everything", did you also watch the fuel pressure after you shut the key off? The fuel system should hold pressure for a while. I ask because.....

makes it sound like you can watch the pressure drop in a few seconds.
If that's the case, you could have some leaky injectors which would be flooding the cylinders, and the engine would run rough at start up until it "clears up".

 

sounds like its running rich sometimes. id say you have a bad connector somewhere. test evap system for piece of mind. check the wires to the 02 sensors. have you tested the egr? gas cap?

 

Check the Mass Flow Sensor. The wires should be silver in color. If they are black, replace the sensor. I was having issues with my car and I just fixed by replacing the MFS.