7.4L MAF Issue?

[Tom P]

So I've got a 2000 K3500. Had the hot start issue and slobbery injectors. Fixed all that and threw in a new thermostat. Its been running great on short distances and has plenty of power. After doing some towing and getting the truck properly warmed up, I start to have some issues with hesitation and even what sounds like a miss at idle.

Does anyone have experience with this issue? No Check Engine light on.

 

[Justin S]

Could be your egr.

 

[Tom P]

Could be your egr.

Not 100% sure but isnt EGR only supposed to kick in at highway speeds? And possibly throw a CEL?

 

[Christian Steffen]

EGR shouldn't activate at idle. What did you use for injectors? I would think if an injector was failing to fire it would throw a code for misfire, but its hard to say. Check to see if you can determine which cylinder it is. Check the plugs.

I did see an issue recently with the MAF on a 2002 impala. The car would cruise along fine then randomly cut out/stumble, it was most noticeable under load but it would do it at idle as well. I data logged the car and it would show the MAF frequency drop to zero then spike to 10000 hz in a very short order.

Try unplugging the MAF then starting the truck, this will illuminate a CEL the second time you start the truck, this should put the truck in speed density (doesn't use MAF) mode so you can confirm if this is a MAF issue.

 

[Tom P]

EGR shouldn't activate at idle. What did you use for injectors? I would think if an injector was failing to fire it would throw a code for misfire, but its hard to say. Check to see if you can determine which cylinder it is. Check the plugs.

Upgraded to the Bosch 4 Hole style. Also installed a delphi fuel pressure regulator and replaced the vacuum hose between the FPR and intake. Incredible fuel economy increase. Went from average of 9 to probably close to 12-15mpg. Will know more at next fill up. Plugs cap and rotor were done right before I purchased... P/O's were chasing the ignition interlock module issue (ignition key security or whatever you really call it) and I have a receipt from the shop that did the work.

Again it only happens when the truck is hot. I would assume injector issue or plug wire cap/rotor issue would present 100% of the time.

One thing leading me to MAF issue is the P/O's had installed a K&N air filter (drop in not cold air kit) and I noticed when changing it the intake tube was oily and smelled like gas, which I assume is from the injector and FPR issue? Pulled the KN and installed a dry paper element.

 

[Christian Steffen]

Definitely get some maf cleaner (make sure it is actual maf cleaner) and try that then. Its crazy how bad K&N are, but yet people still buy them. And you're right, injector or spark wouldn't be intermittent, but I don't think dirty maf would either (still clean it).

 

[Tom P]

One thing I noticed is that when it does finally get to warm up that much, is that the idle finally settles down to around 500 rpm. I dont exactly trust the gauge cluster in my truck as precise due to age, mileage and chronic issues from the factory, but I can assume the tach is close.

Read on another forum it could be O2 sensors, however I've had several vehicles in the past with what i thought were O2 sensor issues, went to the horrific trouble of replacing them to no avail.

I will have to try the MAF unplug test you mentioned and compare results.

 

[Tom P]

Try unplugging the MAF then starting the truck, this will illuminate a CEL the second time you start the truck, this should put the truck in speed density (doesn't use MAF) mode so you can confirm if this is a MAF issue

Finally got a chance to do this. It did throw the CEL so at least I know that system is working properly. However its more confusing because now I have an issue that the computer is not recognizing. Unplugging the MAF didn't seem to change much. I did get to clean the MAF, and took off quite a bit of build up. Its possible that the truck is more responsive after doing that, but it could just be the power of suggestion making me think the truck is running better. It still runs great while cold and then as soon as it hits operating temperature, seems sluggish and will hesitate after idling at a stop light. I suppose driving it longer without the MAF plugged in might be required. I did drive it with the EGR valve disconnected with no apparent improvements.

 

[Christian Steffen]

If it was a MAF issue, it would have been evident once it was unplugged as the tune should switch to SD immediately. This could point to the o2s as you mentioned above. I believe that in the stock tune the computer goes into closed loop (using o2s) at 131*F. It could be that they are going bad, and causing the truck to run lean, which would explain the hesitation and sluggishness.

Amazon has denzo replacements for our trucks for like $30 a piece.

https://www.amazon.com/gp/product/B000C5SG6I/ref=od_aui_detailpages00?ie=UTF8&psc=1

 

[Tom P]

I believe that in the stock tune the computer goes into closed loop (using o2s) at 131*F

This would explain why I didn't notice any hesitation before I replaced the faulty thermostat. O2 sensors it is then. I've read the downstream sensors send signals but do not influence the tune of the engine? Other than the possibility of throwing a code what is the reason for this?