95 Yukon 5.7 Liter V8 Hard Starting When Warm

[Lowufo]

So like the title says I have a 95 Yukon with the 5.7 liter V8 that is hard to re start once the engine is warmed up. Some background on what was going on. It started to run poor so first thing I did was pull codes. I had a EGR code, coolant sensor low temp code and O2 sensor code. So after looking at the engine the connector for the coolant sensor was broken right off so I replaced the sensor. Next I pulled the EGR valve and it was plugged solid so I replaced that. Next I looked at the O2 sensor to change it but received the wrong part. So I hooked the battery back up and truck started fine ran excellent drove like it should and did not through anymore codes so I thought it was fixed. After driving it for a few days it started throwing EGR code again. So had a look to be sure everything was hooked up and I noticed someone at some point had capped off the vent on the EGR solenoid. Since the EGR valve was new and I was still getting EGR code I replaced the EGR solenoid. It no longer throws EGR code or O2 codes but now it throws the coolant sensor low temp code and re starting the truck is very hard when at operating temp. Once it does start it idles fine but as soon as you give it any throttle it falls on its face.
So I did a continuity check on the wires from coolant sensor back to the computer and they both show continuity and they are getting 5 volts. I also checked the ohm reading of the sensor based on engine temp and it is close at 3750 ohms but the chart says it should be 4450 ohms based on engine temp so not sure if the sensor if out of range. I am starting to think the new sensor might be out of range or I got the wrong sensor but don't think so as the sensors looked identical. It seems to me like the issue is the computer is not getting the proper info from the coolant sensor when trying to re start when warm. Its like the sensor is telling the computer the engine is cold so it gives it a rich air fuel mixture making it hard to start. Once started it idles fine and after a few minutes of idling it runs and drives as it should. I am kinda stumped at this point what the problem is. Any ideas where I go from here??

 

[Lowufo]

Bump .....

 

[OR VietVet]

What brand sensor? Should use GM and not junk aftermarket. The thermostat may be sticking open and the engine temp is not right. The age of the ignition module may be the problem as well. The heat sink under the module goes away over time. If you replace it, get a GM module and new squeeze out film heat sink. Age of the cap, rotor, wires and plugs. Age of the fuel filter. Fuel pressure at cold and hot start?

 

[Lowufo]

Sensor is not OEM and I suspect that may be the issue. If I check in various temperatures of water it works but is out of range compared to what the chart says the omh reading should be. I don’t like the connection so I ordered a new connector as well. Double checked the continuity of wires back to PCM and they are good. I will try the new sensor and connector and see what happens. Cap, rotor, plugs are all new and fuel filter has been done recently. The truck runs perfectly from a cold start it is only when the truck is at operating temperature and you try to re start it the problem occurs. Once you do get it started and idle it for a few minutes it runs and drives perfectly. Because it keeps throwing code 15 for the coolant temp sensor I suspect it is out of range or the connection to it is intermittent. Getting the new parts shortly so we will see what happens.

 

[east302]

Seeing what the computer is reporting for the temperature would be worthwhile, but the hard hot-start does sound like a good symptom for the ECT sensor. Holding the gas to the floor when starting (on the 96+ at least) will temporarily stop injector flow and is a workaround.

A PDF of the 1995 manual is here...

https://www.gmt400.com/threads/88-95-service-manuals.43575/

Here is the diagnostic for DTC 15, maybe it will help. I’d also check that resistance when the engine is hot (194-F on the table) - I assume the 3750 was observed when cold.

 

[Lowufo]

Thanks for the manuals I had some of those but not all of them. I am still leaning toward an out of range after market sensor. As per the chart at 15C temp it should be 4450 but I got a reading of 3750 this was in the truck. I pulled the sensor and tested it to see if the values went up and down with different temp water which it did. But when I tested the sensor using different temp water samples the readings were out of range leading me to think the non OEM sensor is skewed. I tested it out of the truck in 10C water and got a reading of 4600 and it should have been 5670.

I have an AC Delco sensor and new connector arriving tomorrow so we will see. In the diagnostic aid it says "the temp sensor resistance value scale may be used to evaluate the possibility of a skewed (mis-scaled) sensor" I think that may be the case here and so when I go to restart it is hard to start because pcm is getting the wrong value from sensor giving it a rich fuel mixture. Holding the peddle to the floor works to start it and after idling for min it runs okay again. It keeps throwing the dct15 code which at least to me indicates a coolant sensor or sensor wiring issue. I am going to back probe the yellow wire once all back together and up to operating temp and see if my voltage has dropped from 5 volts to 1 or less volt like it should.

 

[east302]

Yeah, the resistance values do look suspect. I’d wonder how they compare in hot water to simulate a warm engine. If you had access to a scanner I’d guess that the computer is getting a completely off reading from that sensor.

 

[Fless]

Thanks for the manuals I had some of those but not all of them. I am still leaning toward an out of range after market sensor. As per the chart at 15C temp it should be 4450 but I got a reading of 3750 this was in the truck. I pulled the sensor and tested it to see if the values went up and down with different temp water which it did. But when I tested the sensor using different temp water samples the readings were out of range leading me to think the non OEM sensor is skewed. I tested it out of the truck in 10C water and got a reading of 4600 and it should have been 5670.

I have an AC Delco sensor and new connector arriving tomorrow so we will see. In the diagnostic aid it says "the temp sensor resistance value scale may be used to evaluate the possibility of a skewed (mis-scaled) sensor" I think that may be the case here and so when I go to restart it is hard to start because pcm is getting the wrong value from sensor giving it a rich fuel mixture. Holding the peddle to the floor works to start it and after idling for min it runs okay again. It keeps throwing the dct15 code which at least to me indicates a coolant sensor or sensor wiring issue. I am going to back probe the yellow wire once all back together and up to operating temp and see if my voltage has dropped from 5 volts to 1 or less volt like it should.

So it seems that your current sensor is reporting a HIGHER engine temp than actual, correct? at 10° C actual it reports about 14°?

 

[Lowufo]

So it appears it was a skewed sensor. The Ac Delco sensor came in but the new connector was delayed so I went ahead and swapped sensor and took existing harness apart fixed up the spade terminals and put everything back together. Took it for a test drive and no more dtc15 sensor code or hard start when at operating temperature

It did throw a dtc32 egr code while on the highway but it went out after first start cycle. EGR and EGR solenoid were just changed so not sure why it would throw that code again. I think I will get some new 1/4" vacuum line and change all the 25 year old factory oem hard plastic vacuum line with the push on rubber ends. Maybe one of those factory lines has an issue I can't see. I don't know but very happy my hard start issue has stopped. Now we will see if it throws code 32 again.

 

[Lowufo]

Did some more home work on this code 32. It appears the replacement EGR provided was the wrong one so I have the proper AC Delco one on the way as well as new vacuum lines. Apparently these trucks a very particular about the EGR and non OEM units can be trouble. The proper EGR should be a negative pressure unit and it looks like the one I was given is a positive pressure unit. Once I install the AC Delco one and change the lines I will report back if it corrected the issue.