88 K1500 5.7 sputtering, backfiring occasionally

[dirkhood]

I have an 88 with a newer crate engine with 12,000 miles on it. The transmission was also rebuilt 5,000 miles ago. This was put in when I bought the truck last spring to take my little guys fishing and camping. Recently the pickup started sputtering and backfiring when I step on the gas while it is in drive. It idles fine and the sputtering and backfiring only seems to happen once in a while. It is either after it is warmed up and driven several miles or after it is started back up after having driven it. If you shut it off and start it again, it stops. When the engine was replaced, a new distributer, plugs wires, knock sensor, and o2 sensors were put in. I just changed the fuel filter when this started and it did not stop this behavior. I cleaned the EGR valve thinking it might be sticking, again, no change. I checked all the plugs and they look great, I also am not getting any codes.

I read it might be a MAP sensor, I have one on its way but wondered if I might be on the wrong path. I don't want to throw a lot of money into changing parts that might not be bad. I have always had good luck finding advice and I am very appreciative if anyone has any input. Thanks!

[j cAT]

Originally Posted by dirkhood

I have an 88 with a newer crate engine with 12,000 miles on it. The transmission was also rebuilt 5,000 miles ago. This was put in when I bought the truck last spring to take my little guys fishing and camping. Recently the pickup started sputtering and backfiring when I step on the gas while it is in drive. It idles fine and the sputtering and backfiring only seems to happen once in a while. It is either after it is warmed up and driven several miles or after it is started back up after having driven it. If you shut it off and start it again, it stops. When the engine was replaced, a new distributer, plugs wires, knock sensor, and o2 sensors were put in. I just changed the fuel filter when this started and it did not stop this behavior. I cleaned the EGR valve thinking it might be sticking, again, no change. I checked all the plugs and they look great, I also am not getting any codes.

I read it might be a MAP sensor, I have one on its way but wondered if I might be on the wrong path. I don't want to throw a lot of money into changing parts that might not be bad. I have always had good luck finding advice and I am very appreciative if anyone has any input. Thanks!

alcohol fuels when not used after a few weeks begin to go bad..the water gets into this fuel and then the carb problems engine problems increase...

see what the fuel looks like ..should be clear..not yellow or gray[water] ...

when this fuel separates water/alcohol on bottom this will cause your problem..

adding more alcohol does not help..if the fuel is bad you must drain all fuel and get fresh fuel...stabilizers help keep the fuel from going bad but if water gets into the fuel [condensation] from a tank not filled to 3/4 level this occurs with infrequent useage..

[dirkhood]

Thank you so much for your reply and that makes complete sense. Living in the middle of Nebraska, most of our fuel is ethanol blended. I try to drive the truck daily, usually to the gym a few blocks away but less than 2-3 miles a week. The tank is usually only half full or less. I will check the fuel tomorrow. I appreciate your response.

[j cAT]

Originally Posted by dirkhood

Thank you so much for your reply and that makes complete sense. Living in the middle of Nebraska, most of our fuel is ethanol blended. I try to drive the truck daily, usually to the gym a few blocks away but less than 2-3 miles a week. The tank is usually only half full or less. I will check the fuel tomorrow. I appreciate your response.

the use of alcohol fuels in the US is now manditory...with vehicles seldom used this causes the fuel to go bad , you now no longer can get fuel with no alcohol...owners before would use non alcohol fuel with vehicles/boats not used all that often..


it is possible this law may change ..I hope it does ..with this new fuel its , use it or you loose it.. at 3.oo plus it is very expensive..

I just dumped 7 gallons of bad fuel at the gas station..they used it for a heater they have..

what you can do is check that the fuel cap is secure..fuel tank vent system is working ...all the parts are there and connected..the more air into the tank the more condensation....

[fmx_guy]

it probably wouldnt hurt to go over the plugs and wires coil cap and rotor. parts do go out even if they are new. also take it for a 30 or 40 minute drive something could be just a little clogged. ever so often just driving could clear it up. also ive been using lucas gas treatment in mine and having pretty good results. hope this helps

[dirkhood]

Yesterday I was able to get to this and dropped the fuel tank, cleaned it, and changed the fuel filter. The gas was still clear and did not appear to have any water in it. I put fresh fuel in it and it fired right up and ran great. In fact it ran better then it has in a long time. Because I had it, I changed the MAP sensor as well. Just when I thought I had it fixed, it started cutting out when I gave it gas. I tried driving it around for quite a while hoping it would get better, no luck. It still has not triggered a check engine light.

Its so random and driving me nuts.

[j cAT]

Originally Posted by dirkhood

Yesterday I was able to get to this and dropped the fuel tank, cleaned it, and changed the fuel filter. The gas was still clear and did not appear to have any water in it. I put fresh fuel in it and it fired right up and ran great. In fact it ran better then it has in a long time. Because I had it, I changed the MAP sensor as well. Just when I thought I had it fixed, it started cutting out when I gave it gas. I tried driving it around for quite a while hoping it would get better, no luck. It still has not triggered a check engine light.

Its so random and driving me nuts.

since it did run good for a short time this may be the fuel pump or the carb needs cleaning..if the float or needle valve is sticking or not working correctly dirty rubber / seat worn etc this can be the problem..

alcohol on carb's is a problem...just go to the boating forums and read all those carb issues and how they are rebuilding these components quite often.

[dirkhood]

Thanks guys, I don't know a thing about t.b.i. units, it might be time to bite the bullet and have a pro look at it.

[tblake]

Ohm check both injectors at the top of your throttle body, make sure that they have about the exact same resistance. May want to do this right after the truck starts acting up. Also you can watch the spray pattern.

Otherwise you may want to inspect your cap and rotor.

[jdmccright]

I'd check/clean/replace the IAC valve located on the passenger side of the TBI base. And also check your throttle position sensor...it's fairly rare these fail, but not impossible after all this time.

[EDIT]
Forgot to add the EGR valve under the check/clean/replace. Both this and the IAC can cause stumbling & power loss.

[dirkhood]

Thanks for all the input everyone, we are getting pounded by snow today so it will be the weekend before I get to this again. Again, I appreciate everyones help.

[dirkhood]

Its fixed! After checking all the sensors and cleaning everything, I double checked the cap and rotor despite it being new. The rotor was bad, but and still under warranty. I had them replace the cap as a just in case and she runs like new again. Wanted to come back and thank everyone for your help and guidance.

[j cAT]

Originally Posted by dirkhood

Its fixed! After checking all the sensors and cleaning everything, I double checked the cap and rotor despite it being new. The rotor was bad, but and still under warranty. I had them replace the cap as a just in case and she runs like new again. Wanted to come back and thank everyone for your help and guidance.

thanks for that feedback . If the cap/rotor gets damaged again it may mean the distributor needs replacing [bad bearing] or the wires have high resistance causing arcing to occur inside the cap, instead of going to the plugs. resistance check of the wires would be a good task just in case..

[dirkhood]

I cannot win, the issue is back. My truck ran like a dream for a couple days and now the sputter is back when I step on the gas. It isn't doing it every time you get on it, but it tends to get more consistent the more I run the truck. I pulled the cap off and inspected it and the rotor, they still look brand new. Could this be the ignition coil starting to fail or is it bad wires? The wires look new and I thought they were replaced when the engine was replaced about 11,000 miles ago. They are a heavier duty silicone covered set.

Also, I forgot to note it also is starting miss while sitting idle. It was doing this before, stopped after changing the cap and rotor, and now doing more often than before.

Thanks again for your time

[jdmccright]

Next things I'd check for are vacuum leaks and the PCV valve, tubing, and valve cover grommets.

Pull the PCV valve and shake it. If it is free it'll rattle. If not, shoot the innards with carb cleaner and shake until the solvent comes out clean.

I presume the PCV grommet in the passenger side valve cover is new with the engine and there's another on the opposite side that goes to tube to the backside of the TBI body. Check these for cracks or stiffness. Also check the tubing and rubber ends for cracks...you want a good tight connection to the TBI.

Finally, grab the carb spray and spray around the TBI base and other vacuum lines to check for any gasket leaks...the engine will stumble when you squirt at a leak site.

Hope this helps!