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H3 Need Help with Intermittent rough start up!

Joebot8484

Member
Messages
20
Location
Central NY
Hi Guys.

I have an issue with my 2008 H3 3.7 L with about 114000 miles. This has been going on for a while and I can't seem to track down whats causing the problem. After numerous forum/internet searches and trips to the dealer no one has come up with a solution. The problem occurs after I drive the truck and get it up to operating temperature then shut if off and let it sit of at least an hour (it has done it after sitting for 4 hours). When I go to restart it, it will sputter and bog or some times stall and takes an extra few cranks to start if it stalls. If the truck is cold it will start fine and if its hot and I shut it down and restart it, it will fire up with no problem. I have no codes or engine lights. The dealer has replaced the cam positioning solenoid, they replaced a broken rear exhaust manifold stud I found and even installed a new exhaust manifold and O2 sensor (the latter I did have a light for). I replaced the plugs with the oem ones and put dielectric grease on the coils. I once had a code for small evap leak but I cleared it and it never returned.

Thanks for taking the time to read all this. If anyone has any ideas please share what you think it would be. If I'm missing any info let me know.

Thanks - Joe.
 
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4speedfunk

Well-Known Member
Messages
4,212
Location
Tardville
This might be caused by a leaking injector. When you shut the truck off...the injector fails to close all the way and it slowly drips raw fuel into the combustion chamber. If you have extended crank time this might be due to the pressure bleeding off from the fuel system, and it takes a few seconds to re-pressurize it. Also because the fuel is already sitting in the chamber, it floods the plug and makes for rough, stumbling starts. The late-80s TPI motors (IROC Camaros) were famous for doing this. Sometimes they leaked so much that the fuel drained past the rings into the crank case. Pull your dipstick and give it a sniff to see if it smells like fuel. The fact that it starts good when its cold is also an indication that it may be starting rich.

Try doing a bleed-down test on the fuel system. Most systems have a schrader valve on the fuel rail, and you can buy a gauge that connects to it. I don't know the exact specs for the H3 but, there is a "cranking pressure", a "running pressure" and a "motor-off pressure". All three of these numbers can tell you if have a weak fuel pump, a leaking injector, or a bad pressure regulator. A worn fuel cap can also cause stuff like this.
 

Joebot8484

Member
Messages
20
Location
Central NY
Thanks 4speedfunk, I though this might be a possibility. I have a fuel pressure gauge in the garage, now I just need to find the specs.
 

Joebot8484

Member
Messages
20
Location
Central NY
I've had no luck finding specs on what fuel pressure is suppose to be while the truck is off. I had a buddy plug it into his GM tech 2 and the only real thing we found was a lazy rear O2 sensor (staying rich and responding slow). I swapped it out and my gas mileage has improved but the truck still sputters during a warn start up.
 

3Hummers

Super Moderator
Staff member
Messages
10,398
Location
Central Texas
I would agree that a leaky injector could be the cause of the sputtering. My case was a little different than yours though. Mine would start fine when warm but cough and sputter when it was cold.
 

Joebot8484

Member
Messages
20
Location
Central NY
I found out my H3 was running on the cold side, my temp gauge would read between 1/4 and 1/2 and my hot air wasn't very hot. So I replaced the temp sensor, the thermostat and flushed the coolant yesterday. I now have hot air again and my temp gauge reads just above 1/2 but my problem still persists! Do need to find those specs and see if have a leaking injector...
 

Chook

Well-Known Member
Messages
706
Location
Brisbane (Australia)
I can't find the fuel pressure in the manual either but looking online I see someone recorded 60 PSI (& a reply saying normal range is 50 - 57).

If the problem is leakdown then it is not the pressure itself that is important, it is when you read it does it drop considerably after turning the engine/fuel pump off. If it stays at pressure (or just below) & holds that pressure then there is no leakdown. If after stopping the fuel pressure continues to drop, then fuel pressure (& fuel) is leaking down somewhere (possibly leaking injector)
 

Best4x4

Well-Known Member
Messages
1,042
Location
Vidor, TX
I had a similar issue with my TJ/LJ's and it was from the #3 fuel injector getting hot after shutdown and the fuel basically vaporizes causing a rough start and in some cases a misfire fault. Insulated the injector and the problem never came back. Might be something to check.
 

RamRod

Well-Known Member
Messages
3,914
Location
AB, Canada
interesting thread, I had two rough starts after sitting for a few days, haven't had an issues since but something I will keep an eye on (and this thread)
 

Joebot8484

Member
Messages
20
Location
Central NY
I can't find the fuel pressure in the manual either but looking online I see someone recorded 60 PSI (& a reply saying normal range is 50 - 57).

If the problem is leakdown then it is not the pressure itself that is important, it is when you read it does it drop considerably after turning the engine/fuel pump off. If it stays at pressure (or just below) & holds that pressure then there is no leakdown. If after stopping the fuel pressure continues to drop, then fuel pressure (& fuel) is leaking down somewhere (possibly leaking injector)

I did have my fuel pressure gauge hooked up to the truck awhile back. I remember the pressure while it was running was around 58-60 psi. If fell off a little when I shut the truck off then after 20 minutes it was around 25 psi. I know there is a fuel return so I don't believe the pressure would stay real high but some pressure has to be retained. I might try to clamp the fuel feed and return lines with a gauge and some pressure on it to see if I can isolate and maintain pressure in the fuel rail and injectors.
 

Joebot8484

Member
Messages
20
Location
Central NY
I had a similar issue with my TJ/LJ's and it was from the #3 fuel injector getting hot after shutdown and the fuel basically vaporizes causing a rough start and in some cases a misfire fault. Insulated the injector and the problem never came back. Might be something to check.

Interesting, I'll see if I can get a temperature reading of that area with my heat gun and see if I have a hot spot near an injector.
 

Joebot8484

Member
Messages
20
Location
Central NY
Not much time to work on the truck lately but I did have it hooked to a tech 2 and noticed on every start up #2 and #5 cylinder misfire a few times. I swapped the #5 and #4 coil and will hook it back up to rule out the possibility of bad coils packs. I did talk to a tech who use to work at a hummer dealership and he said it could be the head ( he said he's replaced heads from 06 up to 09 I5's). I did however install my winch.

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Happy Hummer

Well-Known Member
Messages
1,300
Location
Wisconsin
IMHO your mileage may vary. ...
* replacement coils
* replacement injectors
* make sure intake bolts are tight
* make sure hose under air resonator is replaced or verified
* evap solenoid
All these are possible before head . But you can always do a compression test.

Chris
 

Joebot8484

Member
Messages
20
Location
Central NY
Thanks chris, im slowly checking all these things off the list. Now that spring is here im driving my camaro again so the H3 has been on the back burner.

Sent from my SCH-I535 using Tapatalk
 

Joebot8484

Member
Messages
20
Location
Central NY
Well here's a little update. I pulled the intake manifold and replaced the injectors. I also replaced all the coils, the intake manifold gasket and the valve cover gasket (I had a little leak and figured while I had the intake manifold off why not). Most of the intake manifold bolts were loose and what a pain in the a$$ it is to pull that thing off. I found that my alternator was missing one of the top two bolts and installed a new one. My truck still has the rough start on semi warn start ups! The search continues...:gaah:
 
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Hummertrooper

Probationary Member
Messages
2
Location
Virginia
Any luck?

Hi Guys.

I have an issue with my 2008 H3 3.7 L with about 114000 miles. This has been going on for a while and I can't seem to track down whats causing the problem. After numerous forum/internet searches and trips to the dealer no one has come up with a solution. The problem occurs after I drive the truck and get it up to operating temperature then shut if off and let it sit of at least an hour (it has done it after sitting for 4 hours). When I go to restart it, it will sputter and bog or some times stall and takes an extra few cranks to start if it stalls. If the truck is cold it will start fine and if its hot and I shut it down and restart it, it will fire up with no problem. I have no codes or engine lights. The dealer has replaced the cam positioning solenoid, they replaced a broken rear exhaust manifold stud I found and even installed a new exhaust manifold and O2 sensor (the latter I did have a light for). I replaced the plugs with the oem ones and put dielectric grease on the coils. I once had a code for small evap leak but I cleared it and it never returned.

Thanks for taking the time to read all this. If anyone has any ideas please share


Joe,
Any luck? I have exact same problem, it's sooo frustrating. I've tried everything. If you have solved the mystery please share.

Josh
 

Joebot8484

Member
Messages
20
Location
Central NY
Sorry josh i haven't been able to track down the issue. A friend of mine has a gm tech 2 reader so when I get a chance we're gonna hook it up and read what its doing when it rough srarts.

Sent from my SCH-I535 using Tapatalk
 

Hummertrooper

Probationary Member
Messages
2
Location
Virginia
Sorry josh i haven't been able to track down the issue. A friend of mine has a gm tech 2 reader so when I get a chance we're gonna hook it up and read what its doing when it rough srarts.

Sent from my SCH-I535 using Tapatalk

My neighbor has one and we just did it last weekend totally cold start and per everything he saw all was good except cyl 1 did have a misfire or circuit issue. He cleared it and we are going to see if it happens again after 20 cycles or so. I've never had a mil light.
 

4speedfunk

Well-Known Member
Messages
4,212
Location
Tardville
Most likely the loose intake bolts caused a vacuum leak at start-up, and after the motor warmed-up the leak would seal itself. Then the next morning, after it cools down all night, you get the same problem. You're not the first to find loose intake bolts on the 5-cyl...some guys have actually had them fall out. Not sure where else to look, if re-sealing the intake didn't fix the issue. Could be any number of things...MAF-sensor, O2-sensor, fuel-pressure, coil-paks, valves, head (gulp), or ???
 
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Joebot8484

Member
Messages
20
Location
Central NY
Most likely the loose intake bolts caused a vacuum leak at start-up, and after the motor warmed-up the leak would seal itself. Then the next morning, after it cools down all night, you get the same problem. You're not the first to find loose intake bolts on the 5-cyl...some guys have actually had them fall out. Not sure where else to look, if re-sealing the intake didn't fix the issue. Could be any number of things...MAF-sensor, O2-sensor, fuel-pressure, coil-paks, valves, head (gulp), or ???

Yea the intake bolts were loose but that didn't fix the issue. The truck only rough starts when its semi warn, it's never done it on a cold start or a hot start. Injectors, coils, plugs, o2 sensors exhaust manifold and intake gasket, etc have all been replaced. I really just have to data log it and see what its doing but every time I'm at my buddies shop (who has a tech 2) we're working on muscle cars haha.
 
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