View Full Version : 94 tbi hot start
10-06-2007, 10:25 AM
Help Jim. Ikeep getting code 42 from the ecm.Wont start without throttle.Is an automotive ignition module the same as marine?Iput in a new module from an auto parts store but does the same thing.
The manual lists 42 as IC Circuit Open IC Grounded or Grounded Bypass. Unplug the harness from the IC module and ECM to measure resistance from the white wire to ground and from the tan/blk wire to ground. Also, measure resistance from the purple/white to ground with/without the tach connected. Grounding this wire will keep it from starting, too.
Are you using a scan tool? If so, do an OBD system check and clear any codes. Start the motor (if possible) and let it run for two minutes or until a code shows again. If a code shows up, turn the motor OFF, disconnect the ECM harness connectors and measure resistance on pin J2-23 to ground. DVOM only! It should read 3000-6000Ω.
If it reads in this range, using a test light connected to B+, probe ECM harness terminal J2-24. When the probe contacts J2-24, the resistance should go from 3000Ω to under 1000Ω. If it doesn't, make sure the test light is bright. If it is, disconnect the 4 wire connector and if it's still bright, repair a short on the tan/blk wire.
If it does switch from 3KΩ-6KΩ, reconnect all harnesses and start motor. Code/No Code?- Code means that the ECM connections or ECM have a problem. No code means the short/open is intermittent. Look for an intermittent open or short to ground on the white and purple/wht wires.
For these tests, a regular GM service manual has the information.
Won't start without opening the throttle? Did you recently replace the TPS? If so, make sure it's not the one for a Corvette. That one will read WOT on a TBI in the closed throttle position.
10-06-2007, 05:33 PM
Jim.using a code mate.J2-23 reads 3700,resistance falls when test light is on J2-24.light does not go on.restart and get code 42,new ecm?thanks
10-06-2007, 05:35 PM
Jim,also the tps is original,been doing this for a while but this week it got worse.
Disconnect the TPS and see if it starts. Better to have a bad one of those than the ECM. Going WOT is what you do to clear a flooded motor with injection and it won't necessarily throw a code. If it starts with no TPS connected, it won't start easily but it will start. If you have a MC dealer near you, connecting it to the diagnostic computer will cost you but it's a lot faster than using a scan tool and guessing. Actual data and voltages will be seen and someone with that equipment can really go through a lot of diagnostics if they know what they're doing. Did you check the fuel pressure while it's having the starting issues? Also, if the coolant temperature sensor sees full hot, it won't necessarily throw a code but it will be hard to start.
10-06-2007, 11:58 PM
coolant temp sender at 678 ohms,fuel press. is good,injectors are spraying during crank.If I give it just a touch of throttle it starts.A lot of times it start without throttle.Ill try the tps.Thanks.
What was your altitude and air temperature when you read 678Ω? Had you been running the boat when you measured the resistance? 678Ω is just over 140ºF, according to my manual.
10-07-2007, 09:56 AM
altitude-678ft ASL,the boat was warmed up.i`m using the TBI training manual to check things out.this aft I will test the TPS.Checked all grounds&connectors.
10-07-2007, 12:35 PM
Could the IAC be stuck in the close position?:confused:
It could be. Could be dead, or the IAC function of the ECM could have gone away, too. I saw that happen once. Good point.
Wow- 678 ohms at 678' above sea level. Coincidence?
10-07-2007, 04:56 PM
Jim,cold engine today,coolant temp sensor 1876 ohms @67deg.,tps gray to blue wire .690 volts at idle.run till warm and getting code 42.
10-07-2007, 08:44 PM
putting J2 connector back on I noticed J211 almost out of the hole.Pushed it back in and test drove.engine starts right up everytime.still getting code 42 after every start.The MDTC lights up right away,but at least it starts.Tried the code 42 diagnostics and still points to ECM.
Did you clear codes after re-seating the pin? J2-11 shouldn't change anything- that's the green 'Check Gauges' lead. J1-11 is Ignition and that could cause some problems.
I would run it and keep a close eye on everything. If you can, hook it up to a diagnostic computer to verify your findings. Also, connect a timing light and watch for the advance to increase as you rev it. If that works correctly, rap on a motor mount, or something, to check for timing retard. If that also works, you may be OK.
PM Engine Nut and let him know what has been happening- maybe he can shed more light.
10-07-2007, 10:01 PM
Thanks for the help Jim.the only thing about running like this is the timing is being handled by the module not the ECM is this correct?
10-07-2007, 10:03 PM
My mistake Jim,it was J1-11,Idid clear the code 42,after each start.
The timing is controlled by the ECM telling the IC module to advance or retard because of change in RPM, presence of knock, etc. They're in cahoots on that one.
10-08-2007, 02:14 PM
Skied all morning,engine started everytime.