Engine idol problems NEED HELP

[turkeyneck29]

idk if im posting in the right area but here it is. I have a 97 k1500 with a newly installed 98 4.3(out of s10) in it. The engine runs great but the idol is very messed up. As soon as i turn the truck on it goes straight to 2500-3000 rpms then within 5-7 seconds slowly drops down to 700-1000 rpms but OCCASIONALLY starts up right and goes straight to 500-700 rpms. Also when i drive i can press the clutch and it goes up to 2500-3000 rpms but agin OCCASIONALLY will run right and go to 500-700 rpms. When i stop as i slow down it stays at 2500-3000rpm until ive stood still for a few seconds it starts to go down. I get around 9-12 mpg witch sounds horrible for a standerd v6 single cab. As much as i reset the computer the engine light stays on and throws different codes every time i check it. tps censor, egr valve, pintle position 10% off, missfire, knock censor, 02 censor, to lean/rich, its different every time. I changed spark plugs, wires, distributor, fuel injection spider, cleaned the whole block before installing it and a bunch of other stuff that wouldn't affect idol. Since its been installed ive tried 3 different egr valves, cleaning egr port, brand new tps censor, brand new iac censor and checked this thing inside and out for vaccume leaks. Im sure im missing something but ive spent way to much time and money for it to still have a bad idol. The one thing that could affect something is that there is a hairline crack in the intake manifold where the scre is the screw down the air intake but i wouldn't think that would be the problem is it runs good sometimes then dosent run good and on top of this no vaccume leak code is coming up. I need help if anyone has had this same problem.

 

[jdyates]

Check your vacuum lines.


EDIT: my bad I'm an idiot.

 

[96-1500]

I'm not really the guy to answer this question but if I was in your shoes, depending on how big/where the crack is (I'm assuming upper plastic part?) I would try JB-weld on it and see if it makes a difference. If so, replace the manifold.
I would probably do this regardless as you don't want water etc. getting in there.

Hope you get 'er figured out.

 

[Vortec Lover]

engine idol problems huh?

my first suggestion is to remove the object that your engine is worshiping







sorry, not funny.....

 

[Vortec Lover]

Take it in to get tuned/timed

 

[96-1500]

engine idol problems huh?

my first suggestion is to remove the object that your engine is worshiping







sorry, not funny.....

I was going to make that joke but I restrained myself

Take it in to get tuned/timed

Not sure what exactly he'd tune in this situation....

 

[1996k1500]

Which computer are you using? Make sure it is the 4.3L computer from the donor rig(obviously, pointing out the obvious first.). It could also be the MAS. My buddys truck had a freakin seizure until it ran for a couple minutes cause of the MAS. He replaced the MAS and it ran like new.

The mileage might be due to the weight of your rig.

Sorry if this comment is pointless. Just my 2 cents.

 

[96-1500]

The mileage might be due to the weight of your rig.

Should get better than 9-12. 4.3s aren't good on fuel (underpowered) but something is certainly wrong.

He replaced the MAS and it ran like new

Do you mean MAF sensor by any chance? As in Mass Air Flow? Just wondering.

 

[1996k1500]

Yeah the Mass Airflow Sensor. MAS=MAF lol my bad.

And the 4.3l was not an efficient design. Although it should get about 15 to 20 I would imagine.

 

[Garrinchilders]

Take a can of starting fluid and GENTLY spray a couple of VERY small bursts around the hairline crack in the intake manifold with the truck idling normal, if you get a (even the slightest) change in idle, thatd mean that you are sucking un-metered air that could cause high, or erratic idle.

I don't believe you said if you change the ECU when you changed the engine. If you changed the ECU and the donor truck was an automatic, and your truck is a 5-speed, then you will throw all sorts of strange codes as your computer searched for a transmission that isn't there.

I have a 98 RCSB with a 5.0 and a 5 speed, It does the same thing as far as the high idle between gears with the clutch in and when coming to a stop. It drives me crazy but as far as I can tell from many posts that I have read, it unfortunately happens to standard transmission trucks at some point and no one can seem to determine why. I have changed intake gaskets, TPS, MAF sensor, IACV, and done a crank re-lean. None of which helped in the slightest.