I noticed this issue after doing the motor swap in the truck.

ONLY when in nutral or with my foot on the clutch, if I hit the brakes the idle will bounce. Normally its in a pattern, say anywhere from 1 to 5 seconds of normal idling, then the RPMS will drop 3-500 and immediatly bounce back up to idle only to repeat the process.

Interstingly I've found if the truck is in 4wd, this does not happen at all.

So what vacuum system is overridden or changed when the truck is put in 4wd? Any ideas?

 

I remember hearing about an electronic feature when you step on the brakes it adjusts idle. I will post more as I search it. Your's is fuel injected right?

Something about a fuel-cut system when brakes are applied.

http://www.4crawler.com/4x4/TLCA_Tru...ech.04.09.html

this via 4crawler's old post

 

I think your idle is set too low. When you put on the brake it goes lower (too low) and automatically ups the idle. then the idle is high enough that it lets it come back down again, all to repreat the same process. When in 4wd, the computer either ups the idle or richens the mixture (I'm not sure which). This is to lessen the chances of stalling when in 4wd. Thus it doesn't happen in 4wd.
I could be completly wrong though, this is from a combination of different things that I have read and seen in the past. Good luck with it.

 

Actually it is the other way around, if you read the web page above. Idle is set too HIGH, and when the ECU sees the engine RPM above a certain point and the brakes are applied, it assumes you are driving and trying to brake to a stop. So in part to speed up the stop and in part to reduce emissions, the ECU cuts the fuel to the injectors until the engine RPM falls below that fuel cut point. I find that if I bump my idle RPM much above 1000 RPM, I'll get the brake interaction with the idle, if I lower the idle RPM back to 800-900, it'll idle fine. This feature is cut out in 4WD, I suppose since it may cause problems in low range, slow speed off-road driving.

 

I did stay at a holiday Inn a while ago.....

 

Ahhh very likely.

Currently the truck is also suffering from a slight idle issue. When cold it idles too low, so I have the idle cranked up almost all the way. So right now, it will start at around 800RPM idle, but when its warm its upwards of 1400RPM.

So that makes complete sense why I would be having this problem. Much more simple then I was thinking.

Thanks for the help guys. I appreciate it. I will test that out in the AM and let you guys know.