Bad surge @ idle in park/neutral

[xcablb7]

Working on a friends 01 lb7 completely stock and always has been for 200k miles.
@ idle in park or neutral the rpms jump up and down bad. did a log but forgot to save it, rail pressure commanded vs actual is fine rp regulator current is fine but commanded mm3 drops to 0 and injector on time switches from 380 - 0 and back to 380. put it in drive or reverse its fine stays at 380 for ms injector on time and 4.0 commanded mm3 (truck was cold but continues to do this when warm in fact doesnt do it on initial start up)
any ideas ??

 

[kivetts04dmax]

did you data log the injectors? could be a fpr, mines not throwing a code but I think thats what's wrong with mine..

 

[ecc_33]

sounds like a FICM

 

[kivetts04dmax]

sounds like a FICM

Not trying to steal his thread, but I relocated my FICM as well..is there any way to test the wiring?

 

[juddski88]

if you relocated your FICM, there's a good chance your problem is a pin that isn't seating properly, did you solder and heat shrink your connections? what technique did you use?

James, do you have a spare FICM to try?

 

[MACKIN]

Interested to see the fix. My truck does this hot usually after a long pull on the interstate.

 

[kivetts04dmax]

if you relocated your FICM, there's a good chance your problem is a pin that isn't seating properly, did you solder and heat shrink your connections? what technique did you use?

the pins look excellent, I used crimp/heat shrink connectors and I tugged on them and used a torch until they were totally sealed 360* around the wires..I even started the truck, bent and jiggled every wire and connectors into the FICM and still same surge..

 

[xcablb7]

I do have one didn't even think to try it I'll give it a shot, thanks Tim

 

[GMC_2002_Dmax]

2001's the idle speed is set at 600 rpms, on 2002 and later it can be anywhere between 680-715 rpm.

The fuel temp sensor or ect sensor for the computer could be faulty as there are add-on tables for fuel temp as well as ect for fuel pressure among other things......

The best way is a cold start log with efi-live as you have only 90 seconds or so before the temps raise internally in the engine enough to start changing how the add-on tables affect everything.

 

[xcablb7]

Very possible but why wouldn't it do it in drive also?

 

[Mike_S]

Very possible but why wouldn't it do it in drive also?

A load on the engine can usually stabilize the idle. I have also seen exactly what is being described here caused by bad injectors.

 

[MACKIN]

Interesting no matter what is wrong with a lb7 its injectors. Lets see my heated mirror quit, gotta be injectors. Hey my tire went flat, gotta be injectors. WTF

 

[Mike_S]

Interesting no matter what is wrong with a lb7 its injectors. Lets see my heated mirror quit, gotta be injectors. Hey my tire went flat, gotta be injectors. WTF

Lets look at this. If your injectors are hanging open, they will be delivering more fuel than is being comanded, no? This results in the engine idling higher than the ecm would like...make sense? When this happens, the ecm drops pulse width, however there is a minimum puse time below which the comanded pulse drops to 0. Then the rpm drops fast, causing the ecm to bring fuel back in at its regular referenced idle point, only its more than the ecm realizes because the injectors are shot...causing the engine to overshoot its target idle rpm starting the whole sequence over again. No different than the jack monkeys that program them to lope. So, yes, in this case, it can actually be caused by bad injectors.

 

[xcablb7]

Balance rates are all less than 2.0

 

[juddski88]

I have heard of LLY "lope" issues after being run hard and coming down to an idle. I haven't seen it on a LB7. I have had it not want to idle down all the way quite as fast as I wanted it to, but I don't think that's surging. Tony made good points, but I agree, I would think it would also do it in drive if either were the case.

does it have any crank codes or engine speed signal codes?

 

[xcablb7]

It had a 1781 code but the batteries were weak when I started it I figure low voltage caused that

 

[MACKIN]

Lets look at this. If your injectors are hanging open, they will be delivering more fuel than is being comanded, no? This results in the engine idling higher than the ecm would like...make sense? When this happens, the ecm drops pulse width, however there is a minimum puse time below which the comanded pulse drops to 0. Then the rpm drops fast, causing the ecm to bring fuel back in at its regular referenced idle point, only its more than the ecm realizes because the injectors are shot...causing the engine to overshoot its target idle rpm starting the whole sequence over again. No different than the jack monkeys that program them to lope. So, yes, in this case, it can actually be caused by bad injectors.

I know what your saying . What I'm saying is can there ever be a problem that isn't injectors in a LB7? Wasn't meant to be a dig on you or anything and your diagnostic opinion. I'm just wondering what the freak.

 

[Mike_S]

I know what your saying . What I'm saying is can there ever be a problem that isn't injectors in a LB7? Wasn't meant to be a dig on you or anything and your diagnostic opinion. I'm just wondering what the freak.

Didnt mean to jump on you either Makin. Probably just my mood cause i just found my turbo to be on its way out. No worries man. :thumb: but, to answer you question, yeah there are one or two things...my window regulator recently went out cause of the damn window squeege icing up and sticking the window. Dont think that was my injectors. LOL!