How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Sam C. Your Own Question
Sam C.
Sam C., Technician/Shop Manager
Category: Chevy
Satisfied Customers: 792
Experience:  ASE Master, L1 cert, 25 years experience domestic & import
Type Your Chevy Question Here...
Sam C. is online now
A new question is answered every 9 seconds

1998 Chevy: 1500 with a 5.7..misfire code..cap..cant find..problem.

Resolved Question:

I have a 1998 Chevy K-1500 with a 5.7 and auto trans.The last few years i have been setting a engine misfire code mostly when i try to pull my rv. I have changed the plugs,wires,cap,bug,and recently had the GM fuel injecter service done.Its been to the local Chevy dealer numerous times but they cant find the problem.I did install the hypertech power program a few years ago but removed it after it started setting the code.
Submitted: 10 years ago.
Category: Chevy
Expert:  Sam C. replied 10 years ago.

Hi there,

do you know exactly what code it was? a particular cylinder or was it a P0300 random misfire code?


Customer: replied 10 years ago.
It was the po300 code
Expert:  Sam C. replied 10 years ago.

if it is a 300 code that could be a couple of things that come to mind. Usually I see just one injector go bad, and you just had them serviced. All the secondary ignition has been replaced, so we can rule that out. so that leaves:

  • Fuel pressure. should be around 60-66 psi. if it starts to fall off you could have a lean misfire condition causing the 300 code
  • Dirty MAF sensor. I have had a slightly dirty maf give a low reading and cause a lean misfire/ ping while under heavy load, or the MAF is just getting old.
  • restricted exhaust. on those years I have seen a lot of catalytic converters start to break up, and cause a lack of power/phantom misfire code. you will usually see a catalytic efficiency code, so I doubt this is it but it might be an idea. The backpressure should be less than 1.25psi @2500 rpm

Unfortunately the best way to find a problem would be to hook up a scan tool and go for a ride with the trailer attached. I have had to do that to find a problem under heavy load, or I find the steepest grade possible and try to load the engine to duplicate the problem.

Hope this helps



Sam C. and other Chevy Specialists are ready to help you
Expert:  Sam C. replied 10 years ago.

A fellow expert just sent me a TSB if the above recommendations do not help

Subject: SES Light and P0300 When Towing , Cruising Uphill or on Hard Acceleration - kw 4.3 5.0 5.7 accelerate cruise cylinderhead DTC L30 L31 L35 LF6 LU3 P0300 P0301 P0302 P0303 P0304 P0305 P0306 P0307 P0308 #PIP3081 - (07/23/2004)

Models: .


The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.

The vehicle may exhibit a SES Light due to a P0300 and misfire. If the misfire is related to the information below, it will typically happen while cruising uphill, pulling a trailer or on hard acceleration and then stop misfiring shortly after returning to an idle. Typically, cylinders 3,4,5 or 6 will be the ones to experience this.

If the P0300 SI diagnostics did not isolate a concern, the following may help:

Remove the valve cover, valve springs and valve seals on the effected cylinder or cylinders. A small wire tie or rubber bands can be placed in the valve stem keeper groove to prevent the valve from falling into the cylinder. Rotate the valve while moving it up and down in the guide to see if it binds. If a binding valve is found, remove both cylinder heads and use either of the following methods to increase the stem to guide clearance:

  • Send the head out to a machine shop and advise them to increase the valve stem to guide clearance to 0.002 inch per guide.

  • Hone the guide with a 9 mm hone, such as Snap On BCG249, until a clearance of 0.002 inch is obtained. This will usually take about 4 strokes of the hone.

Reassemble the cylinder heads using new valve seals.


Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.

(96 - 02 Chevrolet Astro) and (96 - 02 GMC Safari) and (96 - 00 Old Body Style Chevrolet Suburban) and (96 - 00 Old Body Style Chevrolet Tahoe) and (96 - 02 Chevrolet Express) and (96 - 00 GMC Yukon) and (96 - 00 Chevrolet Old Body Style C/K Truck) and (96 - 00 GMC Old Body Style C/K Truck) and (96 -02 GMC Savana) and (96 - 99 P32 Cab and Chassis) and (96 - 02 Chevrolet S-10) and (99 - 02 Chevrolet Silverado with 4.3L V-6 Engines (96 - 02 GMC Sonoma) and (96 - 02 GMC Sierra with 4.3L V-6 Engines)

Related Chevy Questions