IAC counts high

[dhworkin]

How many teeth on cam gear ? Could be a tooth off.

That could be the problem.

 

[dhworkin]

Hmm,I've installed dozens and never counted the teeth. It's 2:1,that I know. Easiest way to find that out is buy a degree tape and put it on your harmonic balancer then a dial indicator on tip of the #1 intake rocker and see if the valve opening matches the cam card.

I have the cam card and a dial indicator. Can’t find the magnetic base so I’ll get one today. I don’t know the process but I will by the end of the day. Thank you

 

[Road Trip]

PCV port read 17. Motor has a lunati 20080660 cam. I changed the PCV valve and the rattle stopped. I put vacuum on both valves and the old on needed more to open plus it had more side to side play. It’s driving better than it ever has, but the hesitation/lean condition when accelerating has always been a problem. With timing at 0 it is terrible, timing at 10 btdc not good and at 14 btdc it’s almost gone. I’m have no knock counts and it starts easy when hot so I’m going to 16 next, is that normal? Timing is set mechanically and in the calibration with EST disconnect.

Greetings dhworkin,

Welcome to the GMT400 forum.

I don't know the age and/or condition of your harmonic balancer. If it's the stock one, the outer ring may have
slipped in relationship to the inner hub after all these years. (I wish that a small painted witness mark would
be drawn on all harmonic balancers on the face across the ring/elastomer/hub assembly when they were made
so that this error would be immediately obvious.) Or if the harmonic balancer was replaced, how accurate was the
timing mark's position set up/verified during manufacturing?

In English, unless you have used a dial indicator while the heads were off, or verified the timing mark accuracy
using a positive stop device threaded into the spark plug hole, we don't really know just how accurate the timing mark
on the front cover vs the timing mark on the harmonic balancer is.

So it's possible to follow the timing procedure in the manual correctly and still end up with timing numbers that
sound crazy...but the engine is plenty happy, because the spark timing in the combustion chamber is everything,
while the engine has no idea that the timing marks for human consumption are visually inaccurate for whatever reason.

****

And of course if the new cam's specs are such that there's a difference in the timing of the intake valve closing
vis-a-vis the stock cam (affecting the DCR - Dynamic Compression Ratio) ...then the engine may want more
timing *at low rpm* in order to help compensate for this. Having said this, you want to be really careful that
once the motor 'comes on the cam' that you don't have too much timing cranked in, potentially creating
timing-related overstress/reliability issues.

Given all of the above, if it was mine I'd continue to inch up on the initial timing until the driveability is
maximized for the package, BUT at the same time I'd be monitoring the knock counts like a hawk. Who knows?
The engine's apparent desire for more initial timing could be some combo of new cam + harmonic balancer timing mark
clocking error?

And of course in the perfect world all these details would get chased down, accuratized, and a tune that better
matches the needs of the new motor across the spectrum is uploaded into your TBI's computer. (ie: Vortec heads
feature a fast burn combustion chamber as compared to the original TBI heads, so there could be total timing issues
to consider as well.)

Without knowing the specifics the above is just a lot of long distance guesswork. But I share the above just to
give you some possible reasons for what you are seeing.

Good on you for keeping another one of these GMT400s on the road, and best of luck achieving the performance
goals that you have set for yourself.

Let us know what you discover. Every vehicle in here contributes a little more to the overall mosaic of info in this forum.

Cheers --

 

[dhworkin]

I removed the plugs, used a spark plug piston stop and it looks good. Before that I went for a ride and the hesitation happens but it recovers faster (2 seconds). The first 10/15% throttle is better than it’s ever been. If you continue to give more throttle it will go lean. If you hold the throttle steady when it goes lean the RPMs get higher and it will recover. I’ve tuned in AE, VE, spark,disabled knock sensor and tuned in open loop with an emulator and there’s always that hesitation. So I’m going the mechanical route, what next? I have extra 2 Throttle position sensors, 2 coils, distributor and a flow matched set of injectors.

 

[Scooterwrench]

Greetings dhworkin,

Welcome to the GMT400 forum.

I don't know the age and/or condition of your harmonic balancer. If it's the stock one, the outer ring may have
slipped in relationship to the inner hub after all these years. (I wish that a small painted witness mark would
be drawn on all harmonic balancers on the face across the ring/elastomer/hub assembly when they were made
so that this error would be immediately obvious.) Or if the harmonic balancer was replaced, how accurate was the
timing mark's position set up/verified during manufacturing?

In English, unless you have used a dial indicator while the heads were off, or verified the timing mark accuracy
using a positive stop device threaded into the spark plug hole, we don't really know just how accurate the timing mark
on the front cover vs the timing mark on the harmonic balancer is.

So it's possible to follow the timing procedure in the manual correctly and still end up with timing numbers that
sound crazy...but the engine is plenty happy, because the spark timing in the combustion chamber is everything,
while the engine has no idea that the timing marks for the humans are visually inaccurate for whatever reason.

****

And of course if the new cam's specs are such that there's a difference in the timing of the intake valve closing
vis-a-vis the stock cam (affecting the DCR - Dynamic Compression Ratio) ...then the engine may want more
timing *at low rpm* in order to help compensate for this. Having said this, you want to be really careful that
once the motor 'comes on the cam' that you don't have too much timing cranked in, potentially creating
timing-related reliability issues.

Given all of the above, if it was mine I'd continue to inch up on the initial timing until the driveability is
maximized for the package, BUT at the same time I'd be monitoring the knock counts like a hawk. Who knows?
The engine's apparent desire for more initial timing could be some combo of new cam + harmonic balancer timing mark
clocking error?

And of course in the perfect world all these details would get chased down, accuratized, and a tune that better
matches the needs of the new motor across the spectrum is uploaded into your TBI's computer. (ie: Vortec heads
feature a fast burn combustion chamber as compared to the original TBI heads, so there could be total timing issues
to consider as well.)

Without knowing the specifics the above is just a lot of long distance guesswork. But I share the above just to
give you some possible reasons for what you are seeing.

Good on you for keeping another one of these GMT400s on the road, and best of luck achieving the performance
goals that you have set for yourself.

Let us know what you discover. Every vehicle in here contributes a little more to the overall mosaic of info in this forum.

Cheers --

Good point! I hadn't thought of the balancer ring spinning on the hub. I've only seen that happen a couple times om my 40yr career and it was easily detectible by the rubber hanging out between the rim and hub caused by a leaking timing cover seal.

It's easy to make a positive stop by breaking the porcelain out of an old spark plug and cutting off the ground electrode and threading the ID of the valance with a 3/8-24 tap and run a 3/8-24 bolt into the modified valance. To use it remove all the spark plugs and make sure #1 is part way down in the bore. Install the stop and manually rotate the motor until the piston bumps the stop. Mark the balancer at the 0 mark on the scale. Rotate the engine in the opposite direction until the piston bumps the stop again and mark the balancer. Remove the stop. Measure distance between your marks and if the notch in the balancer rim is not right in the middle of your marks the rim has spun. If it's only a degree or two off it could be factory discrepancy in either the notch in the rim or position of the scale on the cover,make a mental note and compensate when setting the ignition timing.

 

[Scooterwrench]

I removed the plugs, used a spark plug piston stop and it looks good. Before that I went for a ride and the hesitation happens but it recovers faster (2 seconds). The first 10/15% throttle is better than it’s ever been. If you continue to give more throttle it will go lean. If you hold the throttle steady when it goes lean the RPMs get higher and it will recover. I’ve tuned in AE, VE, spark,disabled knock sensor and tuned in open loop with an emulator and there’s always that hesitation. So I’m going the mechanical route, what next? I have extra 2 Throttle position sensors, 2 coils, distributor and a flow matched set of injectors.

Fuel pump or filter starving it under load? Have you checked fuel pressure while driving?

 

[Scooterwrench]

I removed the plugs, used a spark plug piston stop and it looks good. Before that I went for a ride and the hesitation happens but it recovers faster (2 seconds). The first 10/15% throttle is better than it’s ever been. If you continue to give more throttle it will go lean. If you hold the throttle steady when it goes lean the RPMs get higher and it will recover. I’ve tuned in AE, VE, spark,disabled knock sensor and tuned in open loop with an emulator and there’s always that hesitation. So I’m going the mechanical route, what next? I have extra 2 Throttle position sensors, 2 coils, distributor and a flow matched set of injectors.

We were typing at the same time.

 

[dhworkin]

Fuel pump or filter starving it under load? Have you checked fuel pressure while driving?

Thanks, I will check that next. I have the fuel pressure regulator blocked and using a areomotive FPR so that’s an easy test. I tested it one year ago after I changed the pump/filter but new stuff can go bad.

 

[Scooterwrench]

Thanks, I will check that next. I have the fuel pressure regulator blocked and using a areomotive FPR so that’s an easy test. I tested it one year ago after I changed the pump/filter but new stuff can go bad.

Or you got some bad gas. A friend of mine called me once to come drag him off the side of the road. Next morning I got out there and discovered his fuel filter was totally clogged. He was diligent about maintenance and I had serviced his truck just a couple weeks before that. Later that day a girl pulled in saying her car wouldn't go over 40 mph,same problem. Both had bought gas at the same station the day before. During that week I must have replaced ten fuel filters and rebuilt a couple carbs. Seems it happens when the storage tanks get low and also when the trucks are dumping fuel. It stirs up the crap that settles in the tanks and if you're fueling while they're dumping you're gonna get some. If I see a tanker dumping fuel I drive on to the next place or wait till the next day to buy fuel there.

 

[dhworkin]

I changed the TPS and the throttle feels much better, but the hesitation is still there. So today I hooked up the fuel pressure gauge, stuck it on the windshield under the wiper blade and drove around. It tested good, even when it went lean and hesitated. It’s running almost perfect. Except when cruising at any speed you have to accelerate onto a highway, that lean spot makes you press the throttle to 80% or more if you want go faster but then the WOT part of the calibration happens and it’s of to the races. I’ve learned to not do that when the roads are wet.

What next? Could it be the pickup coil?