Preformed a scan on my 2007 Hummer H3x as the check engine light was on, the scan did read "multiple fires". Drove it 4 a little n the light went out. Changed the spark plugs but not the coils as they appeared to be ok. Gas mileage increased n so did the power. Preformed an additional scan a few days later ,but still shows a few misfires but less then b4. Any suggestions on what would eliminate misfires readings? I plan to replace all the coils. If not that, then what could it be?

 

You would get a better more accurate response by posting what was the code that came up on the scan rather than your interpretation of what the code was.

Did you have a P0300, and/or a P0301-P0305?

 

Hey,

I just bought my Hummer a few weeks ago so I'm not terribly familiar with it yet.
However, I am a 3rd gen mechanic and I get a ton of vehicles in with multiple misfire codes - mostly because people do what you're doing and chase the problem from least expensive to most expensive until it gets to the point where you've spent several hundred dollars with no results and end up just bringing the car to me.

This is how I normally address it - in fact I'm working on an 04 dodge ram throwing the multiple misfire codes too

First of all, if you still have your old plugs, put them back in and run some seafoam through your fuel line. I normally feed it through the brake booster line but I'm not sure how the vacuum system is set up on this H3 yet.
I say put your old plugs back on because it's going to knock all that junk out of your injectors and it often fouls out your plugs and you'd rather foul out some old ones than have to turn around and buy another set right? Also, clean that MAF sensor if you haven't.

Secondly, since you'll have the intake taken off to access the plugs, clean the throttle body - make sure you get everything plugged back in when you're done. I cleaned mine the first day I had it and didn't realize how crappy the connections were.

If you do both of those things and it's still throwing the code and/or running rough, pull your camshaft position sensor out while it's running. It'll throw a CPS code but if it idles better - replace that sensor.

If it's still messed up, you could be getting exhaust gasses feeding back into the throttle body through vacuum lines. This could mean your catalytic converter is blocked up or something in the exhaust recirculation system is jacked up. I don't think our cars have EGR valves - haven't looked into it.

What kind of scanner are you using? I'd take it to oreilly's or autozone and have them hook up one of the higher dollar scanners to it and make sure it's not throwing some type of exhaust code. I've found some of the cheaper (harbor freight) scanners won't give me O2 sensor codes.

If all of that fails, I'd go with buying new ignition coils - but if it's not throwing specific cylinder codes I'd hate to drop that kind of money without being sure it would solve the problem.

Hope this helps - I hate throwing money at these kind of problems.

 

Thanx 4 the Reply,
Very good feedback and info! I did the seafoam thing prior to placing the new plugs in. A friend of mine that has his own shop later used a snap-on scanner that broke it down as to what spark plugs were suspect. 1, 3 and 5 plugs were causing some misfires but not as much as b4. When he checked the history w/ that snap-on scanner, he did suggest that I get the coil pack as he thinks this should resolve the problem. I did order them as I was at the shop and found a coil pack from an Auto supplier online For $136.95 I did have it 2day'd to me, so I should have them tomorrow by the end of bussiness day. If this clears it up i'll be happy if not, I'll be turning it over to someone w/ more capable hands!! lol....I'll keep you posted, and thanx again for your taking the time to afford Me you input and knowledge.

 

Are you saying you bought "a" coil pack, or all five at that price?

 

Lol agreed.
Here is a ebay link just for quick reference.

http://m.ebay.com/sch/i.html?_nkw=hummer+h3+coil+pack&_pgn=1

I'm hoping it was for 5.

Chris

 

3rd gen mech.? Get yourself some schoolin' and soon you will be 1st gen. lol

Don't guess, Diagnose & Repair Properly!

 

I'm ASE certified and own a shop here in Fort Worth - been doing this my entire life. I can't diagnose it without him bringing to me so I'm giving him some pointers on what he can do to figure it out on his own instead of paying a mechanic $80 an hour to diagnose it. New spark plugs was a cheap guess - ignition coils are a little more expensive of a guess but they both rule out potential problems for when a mechanic may look look at it. Multiple random misfires is a generic code and can be extremely difficult to chase down especially if there are multiple factors like fouled out plugs, bad exhaust recirculation, bad compression etc. I've had customers bring in MRM codes and it be nothing more than a clogged fuel injector and I've had then come in where it's been blown head gasket, blocked cat, fouled plugs, and gunked up fuel rails.

 

Thanx 4 the link. I purchased 5, from 1A auto. But when I put them in the idle was not what it was when I changed my plugs. The misfires are more pronounced. So.....I did get 5 (ignition coils packs) from a dealership in Delaware and saved on taxes. $194.00 w/ an employee discount used the gm friends an family plan!! Gonna change them this evening and see what happens. No Idea what I'll do w/ the ones I purchased from 1A, there might b a sensitivity issue. If this changes doesn't do it then off to the pros!

 

Thanx! I was thinking that! Open a hummer shop!! Lol