Idle air fun

[Jatkat]

Hi all! First post here, I apologize if it is posted to the wrong location.
Long story short- I have been fighting a high idle in my truck since I purchased it about a year ago. It sits at around 1000 RPM, and only drops down to 900 when in gear. I've done all the basics (sprayed starting fluid and propane all around to check for vaccuum leaks, checked timing, ensured that the IAC is at least moving, etc) with no avail. So yesterday I did exactly what you are not supposed to do, and tried adjusting the idle via the screw. I noticed that the idle would not turn down, but would turn up if I went far enough. Did noticed that with the IAC out of circuit, the truck would shoot up to about 1400 RPM at idle. Rehooked everything, and now it just idles crappy. I figure I can screw with it enough to get it back to a smooth idle, but its still way too high.

Any ideas?

Truck is a 95 (TBI) k2500 4wd with a 454.

 

[BPR Speed]

Crimp off all vacuum lines to see if it's a leak in those areas. If it has a brake booster and not a hydro boost you can crimp off the brake booster line to see if the booster diaphram is blown/leaking. Also try safely cover the TB with your hands to see if the engine still runs enough to not die out. If so you have a large vacuum leak and most likely around the intake manifold somewhere. Does is sound like it sucking air anywhere on one side of the engine more than the other? Most importantly though is to check timing before any lean condition is diagnosed. 4 degress BTDC is where TBI 454s are set at with the brown wire at the firewall disconnected. If all else fails check the water temperature sensor and it's wire at the intake to verify your not stuck in closed loop for the computer to sense the engine warming up. If it never comes down on idle from a cold start suspect the temp sensor (not the one in the driver side head). Keep and eye on the O2 sensor for incorrect readings. A simple voltage check can verify proper operation when warm. I know it's a lot to start with so take your time and take notes as you go.

 

[BPR Speed]

Crimp off all vacuum lines to see if it's a leak in those areas. If it has a brake booster and not a hydro boost you can crimp off the brake booster line to see if the booster diaphram is blown/leaking. Also try safely cover the TB with your hands to see if the engine still runs enough to not die out. If so you have a large vacuum leak and most likely around the intake manifold somewhere. Does is sound like it sucking air anywhere on one side of the engine more than the other? Most importantly though is to check timing before any lean condition is diagnosed. 4 degress BTDC is where TBI 454s are set at with the brown wire at the firewall disconnected. If all else fails check the water temperature sensor and it's wire at the intake to verify your not stuck in closed loop for the computer to sense the engine warming up. If it never comes down on idle from a cold start suspect the temp sensor (not the one in the driver side head). Keep and eye on the O2 sensor for incorrect readings. A simple voltage check can verify proper operation when warm. I know it's a lot to start with so take your time and take notes as you go.

 

[Jatkat]

Crimp off all vacuum lines to see if it's a leak in those areas. If it has a brake booster and not a hydro boost you can crimp off the brake booster line to see if the booster diaphram is blown/leaking. Also try safely cover the TB with your hands to see if the engine still runs enough to not die out. If so you have a large vacuum leak and most likely around the intake manifold somewhere. Does is sound like it sucking air anywhere on one side of the engine more than the other? Most importantly though is to check timing before any lean condition is diagnosed. 4 degress BTDC is where TBI 454s are set at with the brown wire at the firewall disconnected. If all else fails check the water temperature sensor and it's wire at the intake to verify your not stuck in closed loop for the computer to sense the engine warming up. If it never comes down on idle from a cold start suspect the temp sensor (not the one in the driver side head). Keep and eye on the O2 sensor for incorrect readings. A simple voltage check can verify proper operation when warm. I know it's a lot to start with so take your time and take notes as you go.

Awesome advice, I'll check the lines and O2. Did the timing at 4 BTDC, though on my truck the wire is in the cab on the passenger side for some reason. I assume it'll be a bit advanced as I had to do the timing at 1000 rpm. I did an intake manifold gasket last week, as I thought that might have been the culprit (was also chasing down a coolant leak). Co-workers are telling me I could just have a faulty IAC... On the warmup front, I know the truck is getting SOME sort of temperature info, as the cold idle vacuum kicker is backing off the throttle within a minute or so of running.

 

[smdk2500]

, though on my truck the wire is in the cab on the passenger side for some reason

Just a FYI that is the normal place for the wire on a 95. Ive heard that some 94's have it there as well.

 

[BPR Speed]

If you checked timing at 1000 RPM and set it at 4 BTDC at a 1000 then the engine will be slightly retarded as the ramp rate of ignition advance goes up with RPM. No worries though, that can be fixed after the fact. The ignition retard will slow the engine down in RPM as the combustion cycle is not optimum. How's the exhaust feel out the back, does it smell as if the engine is running hot/lean. Are the cats and manifolds hot? Do you suspect any exhaust leaks prior to the O2 sensor? Lemme' know what you find. Another easy way to see if the IAC is working through it's full range of motion is to hook it up to your battery posts. You should see the plunger dip inwards as you make the connection. It should be smooth as expected for a plunger. Parts swapping is the easy way out but it leaves you with little cash and a feeling of not knowing what really was the problem and what your money actually fixed. You also mentioned your loosing coolant, these things need to be corrected prior to trouble shooting other issues. The intakes on these 454s are heavy and cumbersome so if you tweaked the intake gasket on install it can cause you some heart ache in the future. I usually apply gasket cinch from edelbrock on the bare clean surface of the head, intake and steel core felpro gasket. Once tacky (about 5 mins) the gasket gets a bead of black RTV around the water ports with the block off tabs installed (if the tabs are in the gasket set) and then placed onto the engine. The gasket will not move after you get the gasket adhesive tacky. As long as you're careful, use plenty of RTV the intake will seal up fine and no coolant should be drawn into the combustion chamber.

 

[PlayingWithTBI]

Put a scanner, not a code reader, on it and see what your TPS, IAC, MAP, etc are doing. After messing with the idle adjusting screw you should redo your base calibration. Follow these instructions to get it right.