Hi Guys, just joined up after snooping in the back ground, can't see an intro section, so appologies if i have missed it.

I'm running a S & P tune port on my motor, and have an intermittant cold start problem, intermitantly from cold it starts and stalls like its not getting extra fuel for cold start once warm up it fine, ive been checking the loom and cts and mat feedback and wiring back to the ecu and what i have at cold is cts 2.93v and mat 4.17v with ign on, to me this doesnt stack up surely if these sensors are basically doing the same job they should surely have a similar voltage, so my question is does anyone know what the voltage should be at cold with ign on at yellow e16 and tan/blk f16. if my thery is right i need a new ecu.
thanks Guys

 

They each use a different pull up resistor value(s) in the ECM. Check the sensor resistance when cold with the connector disconnected. They should match within a few percent.

RBob.

 

The cts and mat are the same thermistors in both these locations.

 

thanks for replys, should have said, it has got new sensors and i have checked the resistance to the sensors with ecm plug disconnected and both check out ok and the same, checked for bad earths/short to ground etc, my thinking is as the sensors seem to operate on 5v the cts is lacking at 2.93 my thinking is the cts (or the ecm if thats where the fault lies) is effectively telling the ecm that the engine aint cold hence causing the start fault when cold.
what do you think?

 

I was under the assumption E16 voltage varies vs coolant temp up to 5v. I have no idea what the at cold is Im thinking 1.5v so In that case a bad ecm. Rbob is the expert on the 730. you said you check the grounds I assume you checked the splice at the map.

 

a Bit of an update really, decided to heat the sensor while checking the supply feed at the ecu, engine clap cold cts 2.73 mat 4.17, cts voltage starts to slowly drop as does the mat because it is also recieving some hot air, but watching the cts as soon as the voltage hits .99v it jumped straight up to 3.74 v. ok from there i tryed 3 different sensors i have that are all good and there is not difference, so i recon my ecm is knackered, what do you think, oh incidently the voltage with cts unplugged is 4.95v.
Still waiting for my aldl lead to arrive so cannot confirm on live data yet how the voltage change is effecting the cts read out.

cheers Bill

 

Those reading seem like the ecm is knackered ! Does it have a warranty ? Make sure the case of the ecm is not grounded. The aldl connectors are of plenty at the junk yard every obd1 gm vehicle 83-95

 

By not grounded you mean earthed to vehicle body? Why should it not be grounded, the ecm body is earthed i guess via the internals this came to light while checking earths etc.
No such luck no warrenty i'm afraid. what i ment was i'm waiting for a computor lead, ie aldl to usb lead.
taken a gamble before the lead arrives and have another ecm on its way to me.
cheers Bill

 

The ecm case should not be grounded to the chassis negative ground. This can cause odd running issues and can even take out the ecm. I was assuming bye your comment that this was not in a thirdgen. The mounting tabs off a cavilier/beretta 88-93 work well.

 

Well that might be the reason then the unit is duff as it has been grounded to the body, i will look at alternative ways to mount it when i get the replacement.
thanks for help
cheers Bill

 

That is normal. As I previously posted the CTS & MAT/IAT use different pull up resistors. With the CTS also having dual pulls ups that the ECM switches between.

At this time your best bet is to use a scan tool so that you can see what the ECM is seeing & using.

RBob.

 

oh dear thats blown my theory then thanks for update, i will post further info depending on what comes first ecm or data lead.
I did however pull out the ecm and open it up, while everything appears to look ok the printed circuit on the flat side of the ecm is all raised and bubbly which looks a bit odd as the otherside is fine, is this normall or a sign of heat damage?

cheers Bill

 

Ok a little update the ecu has come, before i took out the old one i re read the voltage at cts and mat, then fitted the new ecu and reread the volts again, and guess what they are the same or as near as damit can,t say i wasn't warned though.
its starting ok and stays running but thats half the battle its so intermitant and only does it cold. so until i get the lead to read the live data i will see how it goes.
the only other thoughts iv,e had is the IAC although it appears to work ok, i think but then again, has anyone experienced any issues with them before.
thanks for feedback chaps.

cheers Bill

 

so i started her up this morning and guess what yup stalling wtf, its driving me nuts yesterday it started fine, ran till warm ish then today and stalling, i'm going to fit an iac because the only other thing ive got to go on is the induction noise does vary between good starting and bad.
cheers Bill

 

been a while since my last update, but its taken a while to gather the info and work out whats going wrong when it cuts out.
so looking over the live data i can see that the map reading does not change when the engine starts it stays around 100 kpa, it will die out at this point, ign on will restart and straight away the map kpa will drop to around 35 - 45 and run perfect. i can't see anything else that differs between good running and bad (datamaster)
So can anyone give me any ideas as to why this is happening everything has been renewed or changed other than the dizzy and the loom however this has been checked for shorts to earth and cont etc with nothing found.
any thoughts anyone, RBob/

regards Bill

 

The MAP value in the ALDL stream is the actual sensor value (not defaulted due to an error). So, double check the MAP connections. Both the vacuum line and the electrical connector.

If they are OK it is likely that the ECM is bad.

Can also test the MAP by using a vacuum pump and reading the output. The output will be close to 5 volts at barometric pressure at sea level.

And decrease as more vacuum is applied to the port.

RBob.

 

thanks for your thoughts RBob, i have another ecm which i have tried along with a map and still the same results, vac line is ok no leaks although i haven't tried your suggestion which i will. i will T in a vac gauge to the map vac line to see if it differs when the fault occures aswel.
As its only intermitant and when cold i can only assume it linked to the cold startup as the map and cold start sensor are linked via earth return maybe this is where the problem lies.

cheers Bill