Suburban miss fires after going up a hill?

[Redonthehead]

I guess I'm kinda half kidding - its worth a $5 try. Its a fuel additive you can let the engine suck up via the brake vacuum hose and also put in the gasoline. It cleans out the carbon buildup. Do a search for seafoam on most any automotive forum.

[bluephi1914]

Make sure that your Distributor cap is tight. The base that the cap screws into is plastic and will crack and become loose over time. if this happens the truck will misfire and jerk like crazy.

I had to replace the entire distributor because the holes were the cap screws into was cracked and wouldnt allow me to tighten the cap down. i rigged it for a while and that fixed my misfiring, but i just bit the bullet and bought a distributor for it.

[back2life]

what kind of vehicle do u have

[bluephi1914]

i would make sure your dist cap is tight, The dist. cap has plastic holes that the cap screw into and they will crack over time... mine did, and i had to rig the cap so it woldnt move as the rotor was spinning around. once i secured the cap.. it stopped misfiring.. i ended up getting a new distributor just because, but rigging the distributor holes with JB weld so the cap could have someting to screw into worked.

[tooltimetech]

With this fuel system fuel pressure is very important to over come the poppet nozzles in the fuel injectors. If the fuel pressure drops below the poppet nozzle threshold the injector will not flow any fuel, (if fuel pressure is even as high as 45-50 PSI it will not run, usually takes 50-52 PSI to open the poppet nozzles) that is why fuel pressure and fuel volume are so critical. Under heavy load (driving at highway speeds up a hill or under heavy acceleration) the fuel system is being asked to work it's hardest, if there is a weak component that is when it will show up. Check fuel pump volume, fuel pressure regulator, and fuel system's ability to maintain fuel pressure when shut off. If your system does not maintain fuel pressure when shut down or the pump volume is low it should miss under heavy accel. or going up hills at highway speed. After verifying the fuel supply system make sure the injectors are clean. (In the range of $100.00 around here.) $474 wow is that all that was done is clean the injectors? I really like this fuel system, it is just not understood by most because it does not react like all the other electronic fuel injection systems. Last but not least make sure the spark plugs and wires are fine, and yes I prefer OEM. Hope it helps.

[stubbornmike]

on these vortec motors u can not adjust the timing its computerized
however if the distributor has been tweeked a little the computer does not know that and it will cause CROSS firing... take it to a dealer and have it reset to specs..... if you dont beleave me read a chiltons book..]

[Luis100]

I will said that it has to be something related with the intake gasket area because I just have mine change and the same day after pickup the truck from the repair shop on my way home was when I found the problem, before this job (intake gasket) I did never have the problem......Sub 99 K1500 77K miles.

[markdpro]

Originally Posted by 99subhelp

I have a 99 chevy sub with the 5.7 and I had the seal replaced under the intake manifold and now it miss fires when I climb a hill and get to peak it will missfire badly. It will never do it when your climbing the hill or on flat ground but only when the engine has been enganged on the hill and then comes off of a little. It only happens at speeds over 50 MPH. I have replaced the plugs, wires, cap & rotor, and the fuel filter. The dealer wants to replace all these parts again and charge me 500 for it. I need some Ideas. I took it to another mechanic and he sees it misfire with the scope while driving but can't pinpoint it.

Thanks

Tony

I have this exact condition with my 97 yukon. The dealer replaced the intake manifold gasket. Now on high throttle settings like pulling my boat up a long hill all is well until I crest the hill and back off a bit. Then a really heavy misfire situation lasts for about 5 - 10 seconds. I think vacuum leak causing preignition might be the problem??? This week I got a flashing check engine too under the same load conditions. I really don't want to go to the dealer without a better idea. I have replaced plugs HT leads pcv valve and fuel filter. Any ideas??

[markdpro]

Originally Posted by tooltimetech

With this fuel system fuel pressure is very important to over come the poppet nozzles in the fuel injectors. If the fuel pressure drops below the poppet nozzle threshold the injector will not flow any fuel, (if fuel pressure is even as high as 45-50 PSI it will not run, usually takes 50-52 PSI to open the poppet nozzles) that is why fuel pressure and fuel volume are so critical. Under heavy load (driving at highway speeds up a hill or under heavy acceleration) the fuel system is being asked to work it's hardest, if there is a weak component that is when it will show up. Check fuel pump volume, fuel pressure regulator, and fuel system's ability to maintain fuel pressure when shut off. If your system does not maintain fuel pressure when shut down or the pump volume is low it should miss under heavy accel. or going up hills at highway speed. After verifying the fuel supply system make sure the injectors are clean. (In the range of $100.00 around here.) $474 wow is that all that was done is clean the injectors? I really like this fuel system, it is just not understood by most because it does not react like all the other electronic fuel injection systems. Last but not least make sure the spark plugs and wires are fine, and yes I prefer OEM. Hope it helps.

My 97 Yukon has this set of problems. Same engine. The intake manifold gasket started to leak coolant and the dealer pulled off the fuel system to replace it. When I got the truck back( with a new oil pressure sender, water pump and spark leads $1500 total) I had a little induction growl on throttle opening but unless the radio was off I couldn't hear it(can't hear much anyway says my wife) On a fishing trip many weeks later, I pulled my boat up a long hill. The initial throttle up works very well. strong pull and no miss. Its when I back off at the crest of the hill that the miss occurs. The first time it happened I was on cruise but I can reproduce the effect every time I go to a high throttle situation and back off. When I come back on the throttle I briefly have a heavy miss.

My latest fishing trip has escalated the symtom with a flashing check engine light on the initial throttle up but still no miss until the throttle down throttle up cycle as the load is reduced.

Like everyone, I need reliability and I'm worried about getting stalled somewhere but my fear of the dealer's costs and my doubts about their abilities keep me looking for a diagnosis before I try to fix this.

I do all routine maintainance and have replaced the fuel filter and pcv valve and installed new plugs. I also have an OBDII code reader and there are no codes present.

I'm wondering if the heavy load heats up some component that causes preignition in the multiport Fuel injection throttle body. Combined with a vacuum leak could that be the problem?

I'm at the point of abandoning GMC forever. I have a seven year old truck with 54 000 miles on it garage stored and carefully maintained and I am losing trust in it's reliablity

I really appreciate your ideas cause I really need a fix.

[Fireplug]

After reading all the posts on this problem and seeing the same pattern it would seem to me that the problem is in the EGR system.
I am thinking maybe dirt or something has got into the EGR passages ??
Fireplug

Originally Posted by markdpro

My 97 Yukon has this set of problems. Same engine. The intake manifold gasket started to leak coolant and the dealer pulled off the fuel system to replace it. When I got the truck back( with a new oil pressure sender, water pump and spark leads $1500 total) I had a little induction growl on throttle opening but unless the radio was off I couldn't hear it(can't hear much anyway says my wife) On a fishing trip many weeks later, I pulled my boat up a long hill. The initial throttle up works very well. strong pull and no miss. Its when I back off at the crest of the hill that the miss occurs. The first time it happened I was on cruise but I can reproduce the effect every time I go to a high throttle situation and back off. When I come back on the throttle I briefly have a heavy miss.

My latest fishing trip has escalated the symtom with a flashing check engine light on the initial throttle up but still no miss until the throttle down throttle up cycle as the load is reduced.

Like everyone, I need reliability and I'm worried about getting stalled somewhere but my fear of the dealer's costs and my doubts about their abilities keep me looking for a diagnosis before I try to fix this.

I do all routine maintainance and have replaced the fuel filter and pcv valve and installed new plugs. I also have an OBDII code reader and there are no codes present.

I'm wondering if the heavy load heats up some component that causes preignition in the multiport Fuel injection throttle body. Combined with a vacuum leak could that be the problem?

I'm at the point of abandoning GMC forever. I have a seven year old truck with 54 000 miles on it garage stored and carefully maintained and I am losing trust in it's reliablity

I really appreciate your ideas cause I really need a fix.

[LastOne]

My 98 Silverado has had this symptom for a year now. Dealer checked it for free as they did the CSPI to MFI injector swap on a GM policy program. The result is the 5.7's have a known problem with the valves developing an actual mechanical sticking. Mechanic said live with it or rework the heads. Seafoam through vacuum port and marvel mystery at oil change made it a bit better but in the right conditions it shakes like hell!

[Luis100]

OK, after taking back the truck to the shop we discover a engine code showing problems with the Camshaft Position Sensor we unplug the sensor and start the engine and the problems was there like if the sensor was dead, so we repace the sensor and the problem disapear, so I'm very happy!!!!
The check engine light never came up so we didn't know about any codes, we just plug in the scanner and magic the code came and the poblem faound. Just want averybody to know the end of my problem to try to help some.Good Luck!!!!!!

Luis
10 years working on computers
0 years working on cars

[markdpro]

Originally Posted by tooltimetech

With this fuel system fuel pressure is very important to over come the poppet nozzles in the fuel injectors. If the fuel pressure drops below the poppet nozzle threshold the injector will not flow any fuel, (if fuel pressure is even as high as 45-50 PSI it will not run, usually takes 50-52 PSI to open the poppet nozzles) that is why fuel pressure and fuel volume are so critical. Under heavy load (driving at highway speeds up a hill or under heavy acceleration) the fuel system is being asked to work it's hardest, if there is a weak component that is when it will show up. Check fuel pump volume, fuel pressure regulator, and fuel system's ability to maintain fuel pressure when shut off. If your system does not maintain fuel pressure when shut down or the pump volume is low it should miss under heavy accel. or going up hills at highway speed. After verifying the fuel supply system make sure the injectors are clean. (In the range of $100.00 around here.) $474 wow is that all that was done is clean the injectors? I really like this fuel system, it is just not understood by most because it does not react like all the other electronic fuel injection systems. Last but not least make sure the spark plugs and wires are fine, and yes I prefer OEM. Hope it helps.

The thing is, that when the high throttle setting is first applied, even on a long climb, there is no problem. Its only when the throttle is closed and then reapplied that the miss occurs. Is there a fuel pressure regulator that could be sticking in a high flow situation that closes too slowly causing a temporary over pressure???? Going after the fuel pump is beyond my comfort zone since I believe it is in the fuel tank and I wouldn't want to drop the tank in my garage. If there was a regulator on the induction system though I would try that.

[Houston54]

I have experienced this mystery problem on a trip this weekend in my 99 burb. The code that was stored designated a misfire in the #7 cylinder. Felt like the trans was going to exit the bottom of the darn truck.

The burb is in the mechanics this AM. I will post whatever he finds.

[markdpro]

Originally Posted by Houston54

I have experienced this mystery problem on a trip this weekend in my 99 burb. The code that was stored designated a misfire in the #7 cylinder. Felt like the trans was going to exit the bottom of the darn truck.

The burb is in the mechanics this AM. I will post whatever he finds.

Hope he doesn't come up with sticking valves. I'm going to have to be very convinced before I let them start pulling the heads.