miss at idle

[KMott]

Trying to figure out what is causing a miss at idle in my '92. It seems to be worse when the truck is warmed up, and is most noticable when truck is in neutral (when not under any load). The truck runs great going down the road and has lots of power. I recently installed POD injectors, rotated the fuel pin and turned in the main power screw. The truck also has new fuel lines, filter and lift pump. (the truck had the same miss before all of this work)

When i first got the truck i advanced the timing by pushing the pump up as i suspected the timing was late. It seemed to run better, but im wondering if it could be too advanced? Today i jumped 12v from the battery to what i believe is a timing advance (KSB?) and the truck started missing worse. (engine starts shaking) I also noticed that if i rev the truck out in neutral it will pop sometimes. I dont think i have any air leaks as the tank was off and i checked the pickup with clear line for any air leaks and has new supply line from tank to lift pump. Thanks in advance for any help.

Kevin

 

[cerberusiam]

Pretty sure you will eventually find its a worn head and distribution rotor in the injection pump. Just ordered a Rockentech 14 mm head to see if that cures the problem. We have done everything to eliminate pretty much the same problem including head gasket.



We have 2 trucks doing the same general thing, and 5 of the cheaper chinese head\rotor combos that all do it. Have another 92 with more miles than all the others with a Bosch head\rotor and big injectors that has no issues.



We are getting a late fire at idle on 1 cylinder and it doesn't matter what we have done cannot eliminate it. Last resort is new head and rotor.

 

[ckw346]

you have named the exact sympthoms that my 93 1 ton started while returning from the texas coast. have not done anything to it except try to research the problem.



would you please keep us posted with fix.



thanks, charlie

 

[DVolk]

I wonder if thats the issue with my 93 the head on the pump were do you get those new heads at ? Mine lopes etc still so i dont drive it and it revs way too high

 

[cerberusiam]

You can get the a Bosch head for $950 ordered form Germany.



The cheaper XL Chinese version are about $280. We ordered a Rokin-Tek for $300. Changed the problem but did not cure it.



Finally got some solid numbers on the distance of the rotor to the back of the head and that cured the last rolling miss issues we had. Had to change out hydraulic plugs and shims for a specific size to get the miss to go away. Things are particulary finicky at an idle with big injectors.



Both pumps manifested the miss and the head\rotor was shelled on both. One pump went to the injection shop for a check over because of this and it came back the same way. The shop said they really can't find a miss on the test bench and if the pressures and volume check out it should be good. Not! :-laf



We ended up tossing 1 case because there was something wrong with it that was not working. Pulled all the guts to put in another case, set it up per spec with different shims and plug and it worked like a charm.



Could be injectors also. Had 2 sets from different vendors that had leaks and were messing things up. A lot of these shops ar enot particulary careful on the mating surfaces prep and that manifests in several annoying traits.

 

[DVolk]

Drew changed out the gov. shaft on it and did some other changes but some thing is not right with it and it has stock injectors still , Id like the problem fixed , i wish i just had the spring installed and not the pump tweaked how ever i think the pump was junk

from DCP any how

 

[DEdds]

Im fighting the same thing on my 92 with POD injectors we replaced lift pump and VE with a new Bosch pump. the new pump works great on my other truck without PODs but runs like crap in mine with them. Could Our Problem be the PODS has anyone changed injectors to. I am going to try it tonight.

 

[cerberusiam]

If you haven't pulled the POD's apart that could be the problem. Have had a couple of them go bad.

 

[DEdds]

PM me to remind me to tell you guys what I find. I am going to flow check them to double check.

 

[DVolk]

It could be timed wrong and could be fireing on the exhaust stroke

 

[Mhannink]

crack the injector lines and see if its a miss on one cylinder or a random miss, if its random its most likely the pump, a cylinder miss could be anything from that area of the pump to the injector. once that is narrowed down go from there.



a random miss would suggest any number of problems inside the pump, probably easiest to just replace the pump.



a cylinder miss, it would most likely be the injector, though since you had the miss after the injectors were swapped id say its not that. could be in the pump, the delivery valve for starters, really it could be anything inside related to that cylinder. if it is a cylinder miss, check the valve train, a bent push rod could show some of your problems. i had a bolt in cat engine once that held the intake valve open at low rpm making the miss really noticeable, at high rpm though the miss would clear out, the bolt would bounce around and the valve could seal properly, eliminating the miss.



it does seem like your problem is pump related, i would start by backing off the timing to factory, see if it goes away, it could be too much timing. if it doesn't than it is probably internal, head, plunger, delivery valves, etc. its hard to say what is best to do. maybe replace the head, maybe replace the pump.



the last thing i wonder about, you mentioned you adjusted the main power screw, which one was that? there is a full load adjustment screw located near the top rear portion of the pump. that is something that i have always been told not to mess with, it can really screw up the pump, the only time it should be adjusted is when it on the flow bench and you can tune it properly.

 

[RedRamAndy]

This might be completely crazy, but: After finding a miss on a cylinder by cracking the injection line, could you swap that injector with another cylinder to see if your miss follows it? If it does not, it would be pump-related.

 

[Mhannink]

You can swap the injector if the miss follows it its the injector if it stays than its pump