Rough idle when put in reverse

[2drhoeon4s]

Ok guys (sunlit). I have a slight issue... When I start my truck cold it sometimes has a sluggish idle for about 1-2 seconds.. sometimes it starts up strong, but almost always when i put it in reverse, the idle drops around 200-300 and then pops back up almost instantly to around 600-700. It is always in reverse that it does it, i havent tried drive. Never does it at a red light.. no hesitation on acceleration, It has never stalled either, just stumbled and it has never done it more than once per cold start. If i drive it for a bit and shut it off and start it 10 minutes later it doesnt do it. Only when I let it cool all the way down.. I replaced the fuel filter thinking it was that, no luck. any thoughts?

 

[SunlitComet]

The reverse gear aggravates any weak engine issue, especially one where the ignition is misfiring but has good compression. Go over your cap and rotor to look for built up moisture or dirtiness. And also scan for any code in the P18** range that would not trip a light but indicates a possible problem with gear shift analysis in the PCM. Oh and one last thing check you IAC valve mounting for deposits or possible jamming in the valve itself.

 

[2drhoeon4s]

thank you i will start there!!!

 

[2drhoeon4s]

i have codes p0420 and p0430
and i dont have cats btw.

---------- Post added at 02:21 PM ---------- Previous post was at 02:03 PM ----------

any thoughts on if it could be the Crank position sensor?

 

[2drhoeon4s]

Ok I replaced the IAC valve, it was gunked up something awful...replaced my spark plugs.. all looked good, cyl 1 and 7 were a little dark. cap looked good, but rotor had a burn mark on the tip so i replaced. plug wires are good. now it runs worse than before.... has trouble starting, then stalls when put in reverse... and if you tap the throttle it bogs, but if you ease the pedal down it is smooth... also.... for some reason when the rpms are up around 3k it blows hot air out the vents, but when it returns to idle the air goes cold...

 

[SunlitComet]

Did you relearn the IAC valve?

---------- Post added at 05:59 PM ---------- Previous post was at 05:56 PM ----------

Without Scan Tool

1. Start the engine for 20 seconds .
2. Turn OFF the ignition for 10 seconds .
3. Start the engine.
4. Check for the proper idle speed.

---------- Post added at 06:00 PM ---------- Previous post was at 05:59 PM ----------

and air as in HVAC?

 

[2drhoeon4s]

hmm... no i didnt do that.. wasnt aware i needed to. Can I disconnect my neg terminal for a few hours and then do the relearn process again? and yes i am referring to my HVAC.. I also got MAF cleaner and a new PCV valve. just to cover all the bases..

 

[SunlitComet]

Disconnecting the battery will clear codes and reset the fuel trims to a default setting and will redo them selves after driving a bit. May notice performance issues until they are relearned. However that does not automatically relearn the IAC. I would be picky and use a delphi MAF in the there. Other makers of it typically have issues when put on the vortecs. If possible get me a system voltage reading via DMM or more specifically if you can fuse #12 HVAC When it does switch form one temp to another. Also check to ensure you don't have a vacuum leak with the heater core water valve system.

 

[2drhoeon4s]

I cleaned the maf and replaced the PCV valve,, not sure what fixed it, but it runs 1000 times better now. smooth idle and acceleration, only thing is when you are slowing down it seems to be in neutral, rpms are around 600 and when you are idling but moving it is around 550... any thoughts on that? Do i need to run it more to re learn the idle speed higher?

 

[SunlitComet]

Iffy Connection? Who knows as I am not there to see it myself. At least it is better. I would not worry about the rpm right now at the moment. Perform the procedure I mentioned earlier and see what the end result is. And if you kill the battery power to the pcm it may be funky for a short while. Did the hvac temp change on you again? Not sure on the "being in neutral like state when slowing down" yet. I know if on the highway and I put it into neutral the pcm keeps the engine speed in sync with the tranny so as to have a almost unnoticeable shift back into gear. I have the 0411 pcm so I can't recall if it was on that one or the original pcm. What kinda "slow speed from" are we talking?

 

[2drhoeon4s]

well if i let off the gas at like 40 it wont do it, but if I am slowing to a red light, once it hit about 20mph and under, it seems to be around 600 rpms

Every car I have ever had holds around 1K until you come to a complete stop,..

 

[SunlitComet]

For now just try this to verify operation, when you put it in neutral at different speeds, put it back in gear with no feet on the pedals and compare how soft the tranny kicks in. Let me know what happens. If they are all smooth I think the pcm is just syncing to what ever gear would be active considering all the various sensor inputs like speed, TPS and so forth.

 

[2drhoeon4s]

ok when driving, i let off the gas and if i let it off easy it is smooth, but it i pull off the pedal quick it clunks a little, and when i press the pedal to accelerate it is smooth.. also, now when i am slowing down and i get under 15-20mph, the rpms bounce from 800-500-800-500-800-500... any thoughts? truck shifts out great, trans was rebuilt 1k ago. Ran perfect for the first 800 miles after. also voltage drops low when it does the idling, but im thinking that is because the rpms are too low for the alternator to produce a sufficient amount of power.

 

[SunlitComet]

How much voltage drop? From what to what? And how was the weather when it happened.

 

[2drhoeon4s]

14 ish to 12.5 and back.. it only drops voltage when the rpm drops low though.. weather is normal.. 80 deg. no rain.

 

[SunlitComet]

If you shut it down immediately and you still have 12.5 on your battery for a bit then that indicates the charging system can not charge at idle. To many idle loads? Who knows. Get a load test done on the charging system and try it with A/C, stereo and fans off and again with the fans a/c on to where the fans would kick on for sure. WIll narrow down the cause area. While the voltage drop or bigger one could have made your HAVC mode output change position as power got low it should not effect the temperature control door. The alternator regulator might be okay but the brushes or slip rings might be goofed a slight bit.

 

[2drhoeon4s]

only time the voltage drops is when the idle drops below 500rms.... i could care less aboput the temp thing... it actually stopped doing it.. the thing im worried about is the fluctuating idle when slowing down with the brakes applied.. and the occasional sputter when tapping the gas. and for some reason when i drive it for a bit, then pull in and put it in park, it now drops rpms to 300-400 in park for about 5 seconds. And the fact that when you are cruising and let off the gas, the rpms fall to 600-700... even at 50mph +

---------- Post added at 08:43 PM ---------- Previous post was at 08:43 PM ----------

crank sensor? TPS? Vacuum leak in the Brake booster?

 

[SunlitComet]

well for vacuum leaks the engine would try to compensate using data from the forward 02's so it would not stand out as bad like on a carbed engine. You can scan for crank sensor codes but they are type a and would have immediately made the CEL active. TPS is just that it has no actuating ability. The PCM sees the rpm data and has a baseline rpm of at least 500 in the software no matter what the tune engine is like if. It should bump the rpm to at least that with out other input from most other sensors even if it runs like crap. If it is not doing that the PCM has lost the ability to control the IAC, the wiring is goofed somewhere or the IAC is jamming or not working well inside the TB. Ensure the TB is completely cleaned out on all ports(By removing it. If any IAC port is clogged it will kill the idle.), and with the IAC out and in had cycle the key like i mentioned above and see that it moves freely. Do it again to bring the pintle back in and shut the key off as soon as it does. Ensure the o-ring for it is not damaged and reinstall. Do the relearn procedure and see what results. Remember I am not there and "wired in" so I may may repeat something you already did. Or miss something you may have done. You went thru all the grounds including the thermostat cover, fender well and the behind the right cylinder head correct?

 

[2drhoeon4s]

ok i can tell you that i cleaned the TB out right before all of this started... i did not remove the IAC then and when it was mentioned as a cause I removed it and inspected.. it was sticking and full of buildup, I replaced with a new one but didnt clean the area that I installed it into..

When you refer to an IAC port are you talking on the IAC or on the TB?

I will try the IAC inspection tomorrow... and the relearn, also I am getting a heavy fuel smell when i start it and for the first minute or so, not raw gas smell, but super rich running smell.

---------- Post added at 10:04 PM ---------- Previous post was at 10:00 PM ----------

grounds are good...

Not sure I have ever seen a ground at my Tstat cover, but Ill verify. Also I replaced the ALT about 6 months ago, and i do have dual batteries, but I have added additional ground circuits. I figured the NEG usually just takes the path of least resistance so if I gave it plenty of paths it couldnt hurt. I added one from the second batt to the drivers side frame rail by the header, and added one to one of the mounting bolts for the alt bracket. any input on that? neccesary?

 

[SunlitComet]

I mean the port on the TB for the IAC. The thermo cover ground would be this:

Your extra grounds do no harm at all but different things have different grounds hence the mentioning of G103 above. Well the gas smell is a new clue. Where do you smell it from? You said you verified the fuel pressure testing correct?