Random misfires 98 Z-71 5.7L

Hi,
Here is my story, I have been having random misfires in my 98 Chevy Z-71, 5.7L with 118K miles that has gotten worse over the span of 2 years, The Service Engine Soon code "always" says random misfire. It only happens when I am going up a hill and the vibrations throw me all over the cab. I have changed the following:
Fuel pump and filter
Plugs/wires/cap/rotor/coil
Throttle position sensor
Fuel rail/Fuel pressure regulator
Injectors and poppet valves are working good
New K and N air filter
Transmission was rebuilt but I was having this problem before that.

When I get the misfire, if I hit the gas it seems to go away for a moment then come right back!

Any help would be appreicaited! (sp)

same thing happened to me. same exact truck. the oil in my k&n air filter had coated the mas sensor so much that it made my engine run lean all the time. when i would go up a hill the light would come on. this in turn sent to much gas to my two cat. converters thus frying them...to the tune of 1200.00 bucks. 1 new cat, 1 new maf.

I found out that my engine had low compression in 2 cylinders, couple of bent push rods, and some broken vavle springs. So $4500.00 later presto new engine...

'96 C2500 5.7 100K

Truck runs great except for small problem at cruising speeds. Particularly noticable on the freeway: cruising along just fine and I get very subtle random bumps from the engine. It feels like the ignition system turns on and off for a fraction of a second.

I can't duplicate this problem at idle. It sppears to only happen when engine is under a relatively light load. If I give it more throttle engine performs perfectly. Also, this is _less_ noticable when pulling 6500# TT.

Have replaced fuel filter and TPS. Don't know if I should attack the ignition system or the fuel system. Any advise appreciated.

Have you checked the fuel pressure yet? I think I would start there first. Let me know what you find out...

doferty, pull your distributor to check the gear on the end of the shaft, it may be worn. Then it could be that it was pulled out for an intake manifold gasket replacement and never aligned correctly. You could try turning the distributor(V8's only) counterclockwise and see if it gets better. A good scantool will help some-check cam retard or camshaft offset value, it should be 0 degrees +/- 2 degrees on the V8.

doferty, pull your distributor to check the gear on the end of the shaft, it may be worn. Then it could be that it was pulled out for an intake manifold gasket replacement and never aligned correctly. You could try turning the distributor(V8's only) counterclockwise and see if it gets better. A good scantool will help some-check cam retard or camshaft offset value, it should be 0 degrees +/- 2 degrees on the V8.


My truck does the same thing and I checked the distibutor and everything else and everything checked out. I narrowed it down to fuel injectors. I ran Chevron with techron ( same cleaner GM sells) a few times and it quit doing it. Now after thousands of miles it is doing it again so I think I am going to let the dealer clean the injectors for $100.00. They have a sytem that hooks up to the fuel rail and they warm the engine, screw a pressurized bottle on, disable the fuel pump and run the truck off the cleaner until the bottle is empty at which time the engine quits. They then enable the fuel pump and they're done. I have heard it works pretty well and used a form of it back in the 80's when I worked on cars. GM has a problem with it and I have heard in California that they actually have converted them to actual multi-port fuel injection to cure the problem and meet emissions. The trucks currently have sequential fuel injection which doesn't actually control each individual injector. I think it is a cross between TBI and true multi-port. Anyway Gm says the poppet valves in the injectors hang up and cause the mis-fires and fuel injection cleaner with techron is the most effective.

My truck does the same thing and I checked the distibutor and everything else and everything checked out. I narrowed it down to fuel injectors. I ran Chevron with techron ( same cleaner GM sells) a few times and it quit doing it. Now after thousands of miles it is doing it again so I think I am going to let the dealer clean the injectors for $100.00. They have a sytem that hooks up to the fuel rail and they warm the engine, screw a pressurized bottle on, disable the fuel pump and run the truck off the cleaner until the bottle is empty at which time the engine quits. They then enable the fuel pump and they're done. I have heard it works pretty well and used a form of it back in the 80's when I worked on cars. GM has a problem with it and I have heard in California that they actually have converted them to actual multi-port fuel injection to cure the problem and meet emissions. The trucks currently have sequential fuel injection which doesn't actually control each individual injector. I think it is a cross between TBI and true multi-port. Anyway Gm says the poppet valves in the injectors hang up and cause the mis-fires and fuel injection cleaner with techron is the most effective.


I have one these California trucks (1998 Z71 K1500) and there is a extended warranty (10 years or 200,000) on the procedure you just described. If a second cleaning is necessary They change out the SFI for MFI (mutiport flex injection). The funny thing is..that Chevron fuel caused my poppet clogg, where in your case it's the cure.

I have one these California trucks (1998 Z71 K1500) and there is a extended warranty (10 years or 200,000) on the procedure you just described. If a second cleaning is necessary They change out the SFI for MFI (mutiport flex injection). The funny thing is..that Chevron fuel caused my poppet clogg, where in your case it's the cure.


I was talking about fuel injection cleaner and Chevron is the only one I know for sure has Techron. Techron I assume is what makes it clean better. I was told by the Gm dealer to run Chevron fuel too, But just because it is from Chevron and has Techron in it doesn't mean it is good fuel. Techron is something that is added after the fact, When the fuel is being distributed. Most fuel comes from only a few different refineries and the additives added for each store is the only thing different. I just try to run fuel from a station that has a high turn over adn I have had pretty good sucess for the most part. I haven't run any cleaner through in many thousand miles so we will see what happens. Did the dealers injection clean help ?? How long did it last ?? I would be interested in converting mine to the multi-port, But the dealers here in Minnesota have no clue what is even needed to due it. I can't complain because I am at almost 200,000 miles and it has been a great truck.

The dealer's procedure is to blast out the poppets with 150 psi CO2, and then hook up a canister of top end cleaner which is pressureized to 60 psi to simulate the fuel pump pressure. Then add injection cleaner to a full tank of fuel. I found out the top end cleaner is illegal to use in California. So what, they ship my truck to nevada to clean the top end? I don't think so.

The dealer's procedure is to blast out the poppets with 150 psi CO2, and then hook up a canister of top end cleaner which is pressureized to 60 psi to simulate the fuel pump pressure. Then add injection cleaner to a full tank of fuel. I found out the top end cleaner is illegal to use in California. So what, they ship my truck to nevada to clean the top end? I don't think so.

Did it seem to work better when they cleaned it or would I be waisting my money ??

Thunderbolt, pretty easy to convert it over to MFI
93441235 injector assembly with fuel regulator
93442096 bracket
17113206 upper intake manifold gasket set

Cost over the internet was about $260 shipped to my door

Quote:Did it seem to work better when they cleaned it or would I be waisting my money?


I wouldn't know if it works better. I read the procedure from a TSB letter that was sent to me, saying that it would be covered by GM up to 10 years if this condition is in fact caused by the poppets. I can't seem to get them to admitt it's a poppet in # 5 causing the misfire. This is why i'm buying a new truck this week. If I had the fuel meter block adapter and injector coil/balance tester I could do it myself.I got 500 psi CO2 tank with regulator, And Fuel pressure gauge w/ all the fuel line adapter fittings. They said the last time I brought it in, that it was low commpression in #5. Now i'm not a professional mechanic but I do know that low commpression wouldn't be interrmittent.

Thanks skipr. Jeremy was it pretty easy to do once the upper intake was off ?? Does it perform any better ?? I am going to be putting a new long block in the truck in the not to distant future and I am thinking about doing that change then. My truck runs ok, But it has either a bad case of piston slap or a bad wrist pin because when it is cold it rattles like a diesel. It has been doing it for the last 50,000 miles and over time is gradually lasting longer and longer. It goes away when the engine is warmed up so I am assuming it is slop somewhere. I don't like it so I am going to make it last through the winter and drop a long block in it.

Thunderbolt, once the upper intake is off it shouldn't take 30 minutes to swap the injector assemblies. The injectors are labeled on the regulator assembly as to which cylinder to plug them into. There was some performance gain just from the injectors being new and my gas mileage increased about 1 mpg

Thunderbolt, once the upper intake is off it shouldn't take 30 minutes to swap the injector assemblies. The injectors are labeled on the regulator assembly as to which cylinder to plug them into. There was some performance gain just from the injectors being new and my gas mileage increased about 1 mpg


Thanks ! I think I will do that swap when I put the new long block in.

sequential fuel injectors a common cause of misfires... start cheap. tune-up plugs wires cap rotor ect. i highly recomend the use of a cylinder compression tool and a scanner 2 observe sensors and cylinder activity 2 be certain.... 5.7L tend 2 do wierd things due 2 intake manifold gasket leaks 2... spray carb and choke cleaner around the intake, monitor idle rpm

Random missfire is a Knock Sensor !
Please read my story !
I own a 1998 Z/71 5.7 K1500
Started off missfireing on a hill or when i pulled my small trailer.
Service engine soon, dash lite came on, went to a shop pluged up to a hand held unit. it said missfire on 3&5, cleared codes & thought its just tune up time. WRONG !
BEFORE THIS I HAD JUST REPLACED THE FUEL PUMP AND FUEL FILTER THE PUMP HAD GONE OUT.143,000
MILES ON IT, NORMAL REPAIR.
OK BACK TO RANDOM MISSFIRE,
REPLACED SPARK PLUGS ,PLUG WIRES,DIST. CAP ,ROTOR,
Driving around the samething {dash lites up service engine soon }
it RANDOM missfires and shakes loosing power. i return to shop we plug up the hand held unit it says random missfire ? so we think its the coil getting weak, replaced the coil same thing random missfire again.
this is over a 2-3 month period i have to drive the truck easy to work and home like driving Miss. Daisy you kinda get the idea.
someone said the intake gasket is Leaking check it , so with the truck at idle i spray starting fluid all around the intake with no changes in RPM.
NEXT Iam about to do a insurance claim, but i love the truck but its driving me and GM mechanics and all others mechanics up the wall at this time iam looking at a whole lot of money spent already and my truck is not fixed, and thinking about standing on a street with a sighn that reads "will work for repaires on my chevy truck "
ok its time to think again , so i'am putting it all together random missfire no power, piston spark knock, so i called O'Reilly auto parts Dalton Ga. and told jeff my sad story he said to change the Knock sensor
for $32.99 it was worth a try.
part # BWD 98012 FOR MY TRUCK.
PLEASE READ ON !! It fixed the problem its never ran so great , Throttle responce is great!
The knock sensor tells the computer to advance or retard the timing,
it cannot adjust the advance without the knock sensor telling it what to do. The Knock sensor is located on the passenger side of the block easy to get to under the truck on jack stands. but it screws into the water jacket of the block between the freeze plugs you must drain all the antifreeze from the engine before replacing it. The truck runs like a brand new one with 160.000 miles on it 0-100 mph is no problem bring on the HEMI'S ! :smokin: