Truck Misfires When Going Uphill

deaf electrician · March 24, 2010

Hi everyone,

I recently purchased a 1998 gmc sierra with 115k on the clock. Truck runs perfect at all speeds except when accelerating uphill. Then starts to stumble and I get engines service light and po300 and po306. I reset the codes and truck will drive fine till the next time I accelerate aggressively uphill. I can floor it going straight and truck is fine. Only when putting a real load on engine going uphill does it missfire.

I have service records from the previous owner which shows recent tune-up. Cap, plugs, wires, (autolite platinums and carquest wires), Fuel filter. Service records show compression 175/220 (whatever that means). Says scope shows ignition patterns and vacuum wave forms normal. Also shows #3 fuel injector was replaced and new intake gasket installed.

I recently replaced alternator and the battery is new. Previous owner had also installed aftermarket maf senser that was throwing codes, so I put the original back in which he luckly had kept( maybe I should clean it) Is there special cleaner for the maf sensor?

Engine starts fine cold. But occassionally when its hot it hesitates and starts slowly( thinking starter might be going out)

I checked the #6 plug. It appeared pretty white with some ash build up but nothing too major. I have recently installed a k/n air filter and have been using lucas fuel additive for better gas mileage. Deposits could be from the fuel additivel.

I dont have a lot of money to start throwing at parts so I thought I would see what everyone thought. I was thinking maybe just cheap wires and plugs might be problem. Thanks for any input. Eric

Cadrian · March 24, 2010

I have seen on two occasions.... where when you start going up hill you start the chug it was the fuel filter.

but I dont know anything about mechanics of a car...... Just seen this happen twice and the fuel filter fixed it.

Badass69 · March 24, 2010

Hi everyone,
I recently purchased a 1998 gmc sierra with 115k on the clock. Truck runs perfect at all speeds except when accelerating uphill. Then starts to stumble and I get engines service light and po300 and po306. I reset the codes and truck will drive fine till the next time I accelerate aggressively uphill. I can floor it going straight and truck is fine. Only when putting a real load on engine going uphill does it missfire.

I have service records from the previous owner which shows recent tune-up. Cap, plugs, wires, (autolite platinums and carquest wires), Fuel filter. Service records show compression 175/220 (whatever that means). Says scope shows ignition patterns and vacuum wave forms normal. Also shows #3 fuel injector was replaced and new intake gasket installed.

I recently replaced alternator and the battery is new. Previous owner had also installed aftermarket maf senser that was throwing codes, so I put the original back in which he luckly had kept( maybe I should clean it) Is there special cleaner for the maf sensor?

Engine starts fine cold. But occassionally when its hot it hesitates and starts slowly( thinking starter might be going out)

I checked the #6 plug. It appeared pretty white with some ash build up but nothing too major. I have recently installed a k/n air filter and have been using lucas fuel additive for better gas mileage. Deposits could be from the fuel additivel.

I dont have a lot of money to start throwing at parts so I thought I would see what everyone thought. I was thinking maybe just cheap wires and plugs might be problem. Thanks for any input. Eric

Okay you have two codes which are #6 misfire and a random misfire code. Changing the plugs is a simple good start...... but with your other symptoms I bet it is deeper than that. The Gen 1 Vortec has a prominent intake gasket issue and always has. Now if yours are truly replaced they either screwed the job up and you are getting antifreeze in #6 still ( or a cracked head, blown head gasket, but lets stay simple) or they did them and never changed the plugs afterwards. Start with the plugs and wires for now. Watch your coolant tank level and see if it drops.

You can clean MAF sensors with any evaporating product that leaves no residue. Most spray can electrical parts cleaners do just that.

However it sounds more like your problem is fuel related than spark. Especially since under load it is missing. I find it interesting that the PO replaced 1 injector....... Your truck doesn't have injectors. It's CPI and uses a central distribution hub that fires 8 poppet valves sequentially. You can buy individual poppets but the better solution is just to get the MPFI retrofit for it( NAPA, Rock Auto, whoever really, and it's still a Delphi part exactly like the overpriced one from the dealer) if need be.

Let's not get carried away on it though. These trucks have bad distributors ( plastic body), a crappy factory injection setup ( the CPI), along with a fuel pressure regulator that is notorious for failing. Couple that with intake gasket issues and it can get intersting. They are also very fuel pressure sensitive. The pressure needs to be around the 55 psi mark or better otherwise you start have problems........ like hot engine starts. That also can be the regulator, the CPI or the igntion. Fuel filter........ maybe, it's cheap to change if in question.

Stay simple and change the plugs and see if the problem disappears.

BTW, I am not ragging on these trucks, I own one too, a 96 with 29K original. Good luck.

deaf electrician · March 24, 2010

Thanks for the info guys.

Bought ac delco platinum plugs, Maf cleaner, and seafoam. Will clean throttle body, maf and install new plugs tomorrow also going to get Napa beldin premium wires. Also going to see if I can rent a fuel pressure gage to check that as well. Will keep you posted.

vester · March 24, 2010

First, do a tune up with the correct OEM plugs.

2nd get rid of the Lucas and fill with standard or synthetic 5w30. This may not fix your problem, but the Lucas certainly isn't helping.

Keep in mind that this era of truck had intake manifold and fuel pump issues. It would be wise to look there.

jro909 · March 24, 2010

First, do a tune up with the correct OEM plugs.

2nd get rid of the Lucas and fill with standard or synthetic 5w30. This may not fix your problem, but the Lucas certainly isn't helping.

Keep in mind that this era of truck had intake manifold and fuel pump issues. It would be wise to look there.

ditch the lucas, i would recommend a high mileage oil, if your oil conumsptions a little higher then most go to 10 weight unless you live in the yukon or antarctica

deaf electrician · March 24, 2010

I am using the lucas fuel injector octane booster not the oil.

vortec22 · March 24, 2010

My parents had a 1997 Yukon that did similar things. When in Neutral or Park it would run and rev great. When you put a load on it it would hit a wall at around 3000 rpm, sometimes surging above that. I think it set the misfire codes as well. The problem was the fuel filter. Changed it out and the thing ran great again. These things are very sensitive to the fuel pressure.

Ranger482 · March 24, 2010

My 94 had a head gasket leak like was mentioned above, and after about 3,000 miles the #2 cylinder spark plug would get fouled out. It would have a crust on it like antifreeze was building up on it. My truck would do the same thing you described after the plug fouled out. It would be like it would load up and just wanna die if I gave it too much throttle. I'd replace the plug and all would be good again for another 3,000 miles! 3 dollars for a plug or 1,000 dollars to rip the motor apart..... I think I'll just change a plug!

rodram · March 24, 2010

.... just to beat the issue again... I would

- change to ACDelco stock plugs

- change the fuel filter

- check the fuel pressure (spec is 61 to 66 psi)

- get a good fuel injector flush ... vortec injectors can get sticky and not function properly

deaf electrician · March 25, 2010

Here is an update,

Today I took the throttle body off and cleaned it. I cleaned the maf sensor. I used a can of seafoam and dumped it down the the throttle body. I changed the plugs and put in new ac delco platinum plugs. I changed cap a rotor and put in napa premium wires and I put in a new fuel filter.

When taking out the old plugs I noticed that cylinders 1,3,2 had a shiny oil sheen to them and that cylinder 3 was the worst, it was actually a different plug than the others suggestion it had been changed out before. The previous owner had a shop replace #3 injector as well. All other plugs looked fine. Rotor was a little worn but not bad.

So to sum up the truck doesnt drive much better than before. It idles at a little lower rpm and is a little more responsive off the line but thats about it.

Oh and I checked the fuel pressure. 58psi with ignition on and truck not running and it drops to 50ish with the truck running. No fluctuation while revving the engine.

Won't really know if any of this did any good until I go up another hill and put a load on the truck. But it was a good way to spend the day and blow $200 bucks, that I dont have. Oh well at least I know where i stand now as far as what has been replaced.

I will keep you posted if I discover anything useful. Thanks for all the replies and keep them coming. Eric

gcope63 · April 4, 2010

My '97 Yukon kind of has the same issue, its not misfiring but is pinging when climbing a hill under load. I just had the intake manifold gasket replaced and an upper engine clean performed and it helped but I can still hear some pinging when under load, if I downshift it goes away. No codes have been thrown yet though.

Gary

splat10 · April 5, 2010

i have the exact same issue (chugging under heavy load). Done everything you've done too. Got a little better but not completely. right around 3000 rpm is where it starts. Mines a 5.3 vortec though. I change the fual filter and that didn't fix it but it got better. It was real bad before i started tuing it up and it's much better now but still there. I replaced the fuel pump with a cheapo from Advance Auto and i think it may not be giving enough pressure but haven't gotten a gauge to check it yet.

No codes except an occasional EGR code which i think has nothing to do with it. My guesses at this point are either fuel pressure (pump, injectors very dirty, or pressure regulator - no hot start issues though) or Cat converter clogged. Have to figure out where to go now.

rodram · April 6, 2010

.... just to beat the issue again... I would

- change to ACDelco stock plugs

- change the fuel filter

- check the fuel pressure (spec is 61 to 66 psi)

- get a good fuel injector flush ... vortec injectors can get sticky and not function properly

... if your fuel pressure is 58 psi at startup and drops to 50 psi when running then....

... YOU NEED A NEW FUEL PUMP .... spec is 61 to 66 psi in order for the injectors to operate properly...

... get an ACDelco from the dealer, or a Delphi brand from a good parts house... don't bother with any of the other cheaper brands ... they have a bad record of short service life....

... AND get the injectors flushed or cleaned so they operate properly with the new fuel pump...

Chris03Silver · May 8, 2010

hey guys,

this so sounds like what was haunting my truck from the day i bought it.

Pull the fuel rails and then pull the inejctors. I was getting a crazy P0307 code that wouldnt go away. Was driving me crazy. Pulling the rails on the 5.3 is super easy and i got away with not having to buy new o-rings, i had a set incase. But what i found was that some sort of mineral build up in the drivers side fuel rail was working its way to the top of the injectors. Never figured out what the crap was but it was some sort of mineral that only built up on the drivers side rail.. pulled the rail apart from the injectors cleaned them out with MAF cleaner*all i had* and the P0307 code has yet to come back.