2004 GMC yukon Denali hard to start, no check engine lights, missing under load.

boatguy95 · March 12, 2021

Okay super new to this so if its in the wrong place I apologize. Im not a computer guy. My 2004 GMC yukon Denali 6.0 has about 265xxx on the clock. Its developed a starting problem where if it sits more than 3 hours, it will not start unless you flog the gas pedal like an old carbureted truck with no choke. Once it does start it smokes, smells like raw gas, and you basically keep it at high idle until the fan kicks out. If you have already been driving it, fires up just fine. Let me go into what has been done to the truck as i had it freshened up last year.

1. New fuel pump AC DELCO

2. New intake manifold

3. new knock sensors and harnes (AC Delco)

4. Plugs (ac delco)

5. oil pump

6. another fuel pump

7 entire evap system including both purge valves and charcoal canister

8. Evap system lines

9. all grounds for body electrical and PCM

10.plug wires (ac delco)

11. water pump

12. power steering pump

13. alternator

14. battery

15. radiator

16. 3 clutch fans

17. ac compressor and condenser and dryer

18. broken exhaust manifold bolt extraction.

19. pcm re programmed and flashed

20. throttle body cleaning

21. 8 coil packs (AC delco)

22. 8 new fuel injectors (ac delco)

23. timing chain set. timing cover set

24. rear main seal

25 all tranny fluids and differential fluids transfer case etc and filters

I am sure i am forgetting something. I have had this truck to the dealer, local shops, further away dealer, pep boys, even took it to a speed shop in tampa with excellent reviews and all they do is LS. I have had every part replaced at least once with oem parts. I have had this truck a LONG time and it really doesnt owe me anything but it drives nice and it still looks decent. Has no check engine codes. Last weekend it did some reduced engine power message and i limped it to GMC. No codes. Tech said its the cleanest 04 hes ever hooked a computer up to, not even BCM codes for actuators or any of that. Truck runs like dog ******. Uses a ton of gas now. Revs itself up and down upon cold start. Oh yeah brand new throttle body, and mass airflow sensor last weekend. worked great for a day, back to the same ******. Misses at red lights. Fuel pressure regulator is built onto the fuel pump assembly thats why we did it twice even though the values were in parameter, leak down test was fine. No EGR. No fuel filter on the denali( i couldnt believe it either). Pressure is spot on for fuel, running, key on, under load. Almost forgot about the 900.00 gas pedal assembly too. Had a technical done on the harness with tone and resistance, all wires seemed to be okay. Im a boat mechanic, an engine builder we use had the equipment to test the harness on mercruiser(LS GM) and came back fine on resistance on each wire, found two marginal pins in the block temp sensor, they got replaced and new ac delco plug for it. Even had it compression tested(all 8 within 3 lbs and well in parameter) I am out of my mind with this truck. If i wasnt so sentimentally attached to it id never ever put this kind of money into it, but she has been mine since 2007. My ass fits the seat perfectly. We have been married and divorced. If anyone has any other insight on it, I would greatly appreciate it. thanks for your time in reading.... -Chris

Sharpz · March 13, 2021

I would look to your cat/s. After heating up, loss of power, hard to start, excessive fuel use are all symptoms or can be of a bad cat. It might explain why another mech didnt pick up the issue. They just didnt run it long enough for them to get heated up enough.

Sorry disregard my suggestion. I misread you symptoms as having trouble starting after getting hot, not at cold starts.

Having read your post again, I would ask if this starting issue happened before you started replacing items, or if they started after. I think the oddest part reading your issues is that no codes are banking. Surely running too rich or too lean would register as an issue, and throw up a service engine light. The fact that this is a fuel to burn issue (timing) seems to point to a bad sensor or ECM issue.

Edited March 13, 2021 by Sharpz

CamGTP · March 13, 2021

That doesn't make any sense for hard starts when cold, a bad cat isn't going to make the engine flood itself with fuel on a cold start.

This sounds like a tough one though. I'd be curious as to what the MAP sensor reading is with the key on engine off in KPA. Should be near 100 kpa at sea level.

I've seen on later trucks where if the vehicle is flex fuel that the fuel composition reading gets stuck at some really high value even though the owner is running 87 octane or 91 octane fuel. It should read around 10% alcohol content or less you it's not running a blend of E85.

Sharpz · March 13, 2021

4 hours ago, CamGTP said:

That doesn't make any sense for hard starts when cold, a bad cat isn't going to make the engine flood itself with fuel on a cold start.

This sounds like a tough one though. I'd be curious as to what the MAP sensor reading is with the key on engine off in KPA. Should be near 100 kpa at sea level.

I've seen on later trucks where if the vehicle is flex fuel that the fuel composition reading gets stuck at some really high value even though the owner is running 87 octane or 91 octane fuel. It should read around 10% alcohol content or less you it's not running a blend of E85.

yeah I misread his issue, as starting fine, but after 3 hours of driving impossible to start.

bg1988 · March 13, 2021

On 3/12/2021 at 6:17 AM, boatguy95 said:

1. New fuel pump AC DELCO

2. New intake manifold

3. new knock sensors and harnes (AC Delco)

4. Plugs (ac delco)

5. oil pump

6. another fuel pump

7 entire evap system including both purge valves and charcoal canister

8. Evap system lines

9. all grounds for body electrical and PCM

10.plug wires (ac delco)

11. water pump

12. power steering pump

13. alternator

14. battery

15. radiator

16. 3 clutch fans

17. ac compressor and condenser and dryer

18. broken exhaust manifold bolt extraction.

19. pcm re programmed and flashed

20. throttle body cleaning

21. 8 coil packs (AC delco)

22. 8 new fuel injectors (ac delco)

23. timing chain set. timing cover set

24. rear main seal

25 all tranny fluids and differential fluids transfer case etc and filters

MAF/IAT sensors are notorious for failures on the GMT800 i would ask a friend to borrow one

How to Fix a Car that Cranks Too Long (Mass Air Flow Sensor) - YouTube

8 hours ago, CamGTP said:

That doesn't make any sense for hard starts when cold, a bad cat isn't going to make the engine flood itself with fuel on a cold start.

This sounds like a tough one though. I'd be curious as to what the MAP sensor reading is with the key on engine off in KPA. Should be near 100 kpa at sea level.

I've seen on later trucks where if the vehicle is flex fuel that the fuel composition reading gets stuck at some really high value even though the owner is running 87 octane or 91 octane fuel. It should read around 10% alcohol content or less you it's not running a blend of E85.

the "key on" data should match the baro sensor (the other map sensor)...these are two different sensors btw(i learned that the hard way)

the Baro MAP(located under the throttle body*) is a open air sensor and the MAP is on top of the intake manifold*

*may vary

Edited March 14, 2021 by bg1988

CamGTP · March 13, 2021

A 2004 won't have a baro sensor just a map sensor.

bg1988 · March 14, 2021

1 minute ago, CamGTP said:

A 2004 won't have a baro sensor just a map sensor.

good to know mine has one 6.0L 1500 series

CamGTP · March 14, 2021

2007+ with the new body style will have a MAP/Baro sensors.

bg1988 · March 14, 2021

1 hour ago, CamGTP said:

2007+ with the new body style will have a MAP/Baro sensors.

the MAF data would be nice too... besides the map data

but the odds are HIGHLY stacked that it's the MAF sensor...

I have a brand new sensor i;ll see where they should be at i.e 600 rpm 1100 rpm and 2000 , 2500 rpm (not any higher as i don't want to "keep accelerating " as the engine is geared way differently to other 6.0s)

between 1200 and 2500 it will be at 20 to 40 g/s(more if accelerating less if steady speed)

Edited March 14, 2021 by bg1988

boatguy95 · March 15, 2021

Update: So even though the new maf sensor is 8 days old(AC Delco, dealer installed), I put the old one back on, started the same. I also took the one off my girlfriends 04 sierra with the 5.3, same thing when i went to start it this morning. Put my 8 day old MAF on her truck, it runs fine no check engine lights i actually drove it to work today instead of the Denali. The guy at buick/gmc was convinced that even though there wasnt a code for the MAF that 9 out of 10 times its the MAF. So i bought one and had them install it when the did the new throttle body assembly. I guess im the 1 out of 10. NEW PROBLEM THOUGH. Yesterday(sunday) after swapping the maf sensor back to the old one, I got a message on the dash that the rear access was open. Lights would stay on unless dome override. Pulled the panel off the gate and checked the glass and door switches with multi meter, all seemed fine and still interrupted the continuity test. Tried to bypass them with no luck. Put the gate back together. Could this be a failing BCM that is messing with the pcm? Its never done anything goofy electrical really. I had to replace the blend door actuator drivers side and a headlight switch back in 2010, but other than basic maintenance stuff battery alternator etc. Its been pretty good for gm electric wise. Im at a loss and it seems to be getting progressively worse. My next thought is new PCM and BCM, but damn when does the bleeding stop? It also started as a symptom thats got progressively worse. Use to roll once then fire up, then twice then start, then cranking with the auto start while flogging the gas pedal. About 3 months to the point its at now. Just keeps getting worse and worse.

Edited March 15, 2021 by boatguy95

bg1988 · March 16, 2021

what fuel do you get? i.e brand

boatguy95 · March 18, 2021

UPDATE!!!! So i finally resigned to that fact that im just gonna have to get out my wallet. I was taking the truck to have it looked at when my temp gauge started climbing at a red light. Usually stays a needle below 210. it was about 2 needles past 210 and its pretty unusual for that truck. Got to the repair shop that services fleet trucks and started explaining everything thats been done, they symptoms, as well as the new running warm symptom. The tech took it for a drive and it was acting stupid, luckily. I saw the tech talking to another older tech while they had their laptop hooked up. Older guy walked away and came back with something brass in his hands. both techs spend not even 10 minutes under the hood and i see the young guy close the hood. My truck leaves and dude is gone for 45 minutes with it. I am fearing the worst this time fumbling for my insurance card trying to find the roadside assistance to get it towed back. He pulls back in and gets out with his paper floor mat in hand. Come inside says something to the service writer and he looks at me and says its done. Engine coolant temperature sensor. $30.00 part. They broke the pigtail so they had to replace that too. Ran great on the way home took it for a 2 hr drive last night just to be sure. Had all my ponies back and didnt miss a lick. This morning from cold start it fired right up on the first key bump and so far so good. Anyone ever hear of a temp sender causing so much havoc? To be specific it was on the drivers side cylinder head near the #1 cylinder. Right by the spark plug. Even my "Rear Access Open" went away. Gas mileage on the road was about 17 mpg. Fingers crossed that it stays that way but so far so good. Thank you guys for the insight and the help. I really do appreciate it.

CamGTP · March 18, 2021

I've seen them go haywire before. That temp sensor gives a reading to the PCM and the cluster. I've seen the temp sensors stick at certain values or read negative temps when it's really 60 degrees outside. Something like that will cause problems with starting and running.

bg1988 · March 29, 2021

On 3/15/2021 at 6:25 AM, boatguy95 said:

Update: So even though the new maf sensor is 8 days old(AC Delco, dealer installed), I put the old one back on, started the same. I also took the one off my girlfriends 04 sierra with the 5.3, same thing when i went to start it this morning. Put my 8 day old MAF on her truck, it runs fine no check engine lights i actually drove it to work today instead of the Denali. The guy at buick/gmc was convinced that even though there wasnt a code for the MAF that 9 out of 10 times its the MAF. So i bought one and had them install it when the did the new throttle body assembly. I guess im the 1 out of 10. NEW PROBLEM THOUGH. Yesterday(sunday) after swapping the maf sensor back to the old one, I got a message on the dash that the rear access was open. Lights would stay on unless dome override. Pulled the panel off the gate and checked the glass and door switches with multi meter, all seemed fine and still interrupted the continuity test. Tried to bypass them with no luck. Put the gate back together. Could this be a failing BCM that is messing with the pcm? Its never done anything goofy electrical really. I had to replace the blend door actuator drivers side and a headlight switch back in 2010, but other than basic maintenance stuff battery alternator etc. Its been pretty good for gm electric wise. Im at a loss and it seems to be getting progressively worse. My next thought is new PCM and BCM, but damn when does the bleeding stop? It also started as a symptom thats got progressively worse. Use to roll once then fire up, then twice then start, then cranking with the auto start while flogging the gas pedal. About 3 months to the point its at now. Just keeps getting worse and worse.

Ground wires if you hatched opened sounds like power was applied to it.. or power was rerouted tripping the sensor check all the ground wires ...