Rough idle with big cam

I'm tuning a friends 6speed '95 LT1's idle. He's got a big cam but it's pretty bad. I increased idle rpm to 900rpm and that helped a bit. Has anyone experimented with idle timing for big cams? IAC adjustments? any other ideas or tips?

barometric pressure here is 86kpa
his map at idle is 53kpa
for reference, my stock engine is ~32kpa at idle

Also has a nice surge at low rpm under some load (like pulling away from a street light).

Not a big cam but for my 218/228 112 LSA Hot Cam. Cranked TPS stop setscrew a bit to get TPS=0% at .75volts, I have an A4 and upped the idle to 800 D/ 900 N/P as you did, also added 3 degrees to the "Closed TPS Spark Advance Vs RPM" table in the 400-1200 RPM ranges, then tweaked the main spark table on the 400-1200 ranges to match TPS closed settings so it transitioned out of 0 TPS smoothly. I stayed away from drilling holes in the TB but I hear it is sometimes necessary w/big cams if you can't get enough air past the TB plate w/o cranking the TPS stops past .75 volts. My idle MAP at 900 is now about 48-50 kpa that's as low as I could get it.

Set the idle timing for the 400-800-1200 rpm cells to 32 degrees sometimes 34 works better have to try it to see. Zero out the target idle over/under speed advance/retard tables for a couple hundred rpm around 0. I would also adjust the tb blades so the iac was in the 80-100 counts range.

As for the surging, you could change the o2 sensor switch points for low airflows to fatten it up and add some timing after looking over logs, modify where the blm cell boundaries are so that it can adjust itself. Usually what happens is the car will end up with lean missfires due to high load and low rpm and a pretty lean air fuel ratio.

Last thing is he will have to adjust his driving to keep the revs up and stay in gears longer. A cam of any decent size is gonna have issues under 1500rpm with any load.

I had to set my idle at 1000 rpm for it to stay running. Especially when it's cold.

So how do you know how big of a hole to drill in the throttle body? and where?
He has a 58mm TB.
His closed throttle position voltage is .63v, so just adjust the idle set screw till its like .75v?
It's a big blower cam, .580 or .590 total valve lift if I recall correctly


Also what are the IAC "counts"? are these like individual steps from a stepper motor or something? and what settings are idel for it?

You will need to either post a datamaster data file of the problems you mention or look at items such as timing since if you dont adjust timing on the new cam from the stock tune then it will run very bad at idle and just off idle.

Bigger cams need much more timing at idle and low RPM than stock tunes, ontop of this the MAP readings at idle are much higher than the stock cam so this adds even further reduced timing to the idle/low load areas.

I could not believe how bad mine ran with stockish timing from just a cam swap, increasing timing to around 34 degrees at 850rpm on up made huge difference and make sure the timing curves are smooth on takeoff or wherever your problems are, if you see a ~5 or more degree sudden change in timing right at the surge area then that is likely your problem. The smoother I made the timing curves (as seen in datamaster) the smoother the motor ran.

Aside from this, with my cam I had to finally run Open loop due to the 02's reading false rich and making the motor run lean under light accell, creating slight surging on accel from a stop, and increasingly worse bucking/surging around 30-45mph. I have a manual trans alum flywheel so this is very noticable.

In open loop all accel surging is gone and 30-45mph bucking is just about eliminated.

My cam is around 230 intake, 234 exhaust.

Some other tables to reduce at idle are "timing advance/retard" over under engine speed or something like that. A bumpy idle and stock values here make your idle timing jump all over the place.

Don't drill a hole in the throttle body totally not needed. Adjust the stop screw (do it with the car off, think cut and check).

What your tps voltage is, dosent really matter as long as its not too high or too low (out of range).

Counts are related to the position of the stepper motor in the idle air controller basicly it goes from 0-160 the farther toward 160 it is the more air is traveling through the idle air controller. Stock usually runs about 30 counts. I like to run more with a cammed car, the iac air passes through a series of small passages and feeds air directly to the ports. Getting the counts close to 100 usually helps quite a lot with distribution problems of the lt1 intake when exacerbated by a large cam.

I just noticed his IAC position in datamasters (at idle) is Zero.
Occasionally it'll bump up as high as 4.

In comparing my car (stock motor), it's more like 30.
Does this mean his IAC isnt making any adjustments and the throttle blades must be cracked open, even though the TPS reading is 0% (at .63v)?

Does setting the idle setscrew past .75v register 1% TPS or something?

Also he has an A/C compressor delete pulley. Should I zero out the "IAC Offset for A/C Anticipate Vs. A/C Pressure" table? (so it doesn't effect anything when his A/C request goes on)

Does setting the idle setscrew past .75v register 1% TPS or something?

The PCM looks at what the TPS voltage is at key on or when you start the car, one of the two and makes that 0%. Im sure it has to be within a certain range or it will throw a code and not reset to 0% any longer.

As long as your getting no codes and the TPS % is changing with every little bit of throttle opening then your ok. Sometimes you canont check this unless the motor is running since the PCM does some learning on that but not exacly sure when, I thought at key on or crank.

If his counts are zero it means the air is either all passing through the throttle body, or he has a vacuum leak (iac valve is closed completely). The solution is to close down the throttle body blades.

Stop worrying about the tps volts its inconsequential as long as its in range whatever the voltage is when the key is on the pcm assumes as zero. Setting it past 0.75 sometimes makes the car think its recieving input, that is out of the range the pcm deems things are acting 100% normal and may or may not cause some erratic behavior. It would be of more concern if you had an a4 to deal with.

I would zero the iac a/c pressure anticipation if its been eliminated. I would also zero the timing table for egr if thats been eliminated.

Awesome. :metal: Thanks guys.

I reset the IAC counts to 115 because thats where it was going after I closed his idle setscrew. I set Closed TPS timing to 34 degrees. Running great now at idle.

Still have a slight surging problem under cruising and slight accelration. I think i'm going to try and minimize the issue through adjusting the MAF calibration tables.

FYI, I have my idle at 42 degrees to get the best vacuum. This gives you a comparison. Further, I completely closed off my throttle blades and drilled out my TB to get my IAC to run around 40 or so when warm.

On big cams, cracking the TB will work but the best way to do it is to have ALL idle air running through the idle plenum NOT the main plenum. This evens out BLM's on Right vs. Left bank at idle. Mine are perfect at idle even with my VERY big cam.

FYI, I have my idle at 42 degrees to get the best vacuum. This gives you a comparison. Further, I completely closed off my throttle blades and drilled out my TB to get my IAC to run around 40 or so when warm.

On big cams, cracking the TB will work but the best way to do it is to have ALL idle air running through the idle plenum NOT the main plenum. This evens out BLM's on Right vs. Left bank at idle. Mine are perfect at idle even with my VERY big cam.

whoa 42 at idle. what do your timing tables look like? you must be running a lot of timing after 1000rpm.

FYI, I have my idle at 42 degrees to get the best vacuum. This gives you a comparison. Further, I completely closed off my throttle blades and drilled out my TB to get my IAC to run around 40 or so when warm.

On big cams, cracking the TB will work but the best way to do it is to have ALL idle air running through the idle plenum NOT the main plenum. This evens out BLM's on Right vs. Left bank at idle. Mine are perfect at idle even with my VERY big cam.

Yes but you're talking about drilling a bleed into the iac plenum not drilling a hole in the blades. I instructed him to shut the blades to force the iac counts up so more air would flow through the iac passages rather than past the blades.

42 degrees seems a little wacky on the idle timing but if it works for you I suppose thats fine are you still running closed loop?

The surging can also be fixed somewhat by reworking the blm cell boundaries. Sometimes there is just an issue with the amount of blm adjustment that can be handled by resetting the boundaries.

Well I got a call from the guy saying the surging/bucking has gotten much worse.
He said he almost got T-boned making a left hand turn when he punched it and it stalled and bucked, and then took off.

I'm just about all out of ideas at this point. I don't really want to disable closed loop altogether, cuz that's kind of scary. I'd like there to be some fueling feedback just to be safe.

Exactly what is causing the surging? Is it from the o2's seeing fresh air due to the cam overlap?

Any ideas..?

Its from being lean and probably random lean misfires. When i have seen this problem and had my wideband on the car the surging goes in perfect time with the wideband, its banging along from about 15-16.5 a/f ratio most of the time on the ones I have seen surging.

If you rework the blm cells it will alow it to better adjust, you can also change the o2 switch points to make it go richer.

You also need to blend the spark timing from idle into the 1000rpm on up cells in the timing map. You're sitting there idling at 34 degrees, then you hit the gas and the timing jerks back to 20 degrees.. yeah that will perform great, or it will fall on its face and want to stall :p. Same things happens to guys who want to launch thier six speed from 4k rpm with no load then the clutch hits and drags the motor down and they drop 20 degrees of spark timing too BOGGA.

This kind of thing never happened with old school hei's but because we can do whatever we like with the timing tables with the opti we have to begin to deal with blending the timing. If you look in tuner cats at the GRAPH of the timing table and blend it into your idle advance stuff after that the smoother you make the timing tables the smoother the engine will run.

Dont get frustrated and quit its just your inexperience showing. Tuning an injected engine completely is neither quick nor easy. Hope you packed a lunch you will be at it awhile if its your first one.

Post a datamaster log, I think I said that before. We can guess all we want or we can look at the file and hopefully see what's going on based on the datamaster logged 02 sensors and timing values.

In the meantime run it in open loop see what happens, if it runs crap only in closed loop then you know your base tune/MAF/ignition, ect are ok and then you can focus on why the 02's are a problem. If the base tune runs like crap try tuning in the VE tables some and see if it improves, it should run great in open loop when tuned properly.

When you run in open loop make sure you reset the PCM before driving it so your not running on any of the previous learnings (resets automatically by uploading a file).


Get it running good in open loop and drive for a few days get the feel of it then switch on closed loop and drive for a few days, at first closed loop should run as good as open loop did, gradually it may begin reverting back to old habits and run like shiznit, if so switch back to open loop and if it runs great again then this is proof your closed loop is messed up.

My closed loop is messed up so I threw it out.


Lots of thoeries on why overlap hurts 02 readings, from what I gather excess unburnt fuel vapor in the exhaust is messing up the 02's ability to read correctly.

Lots of GM prototype/test hot rod engines like those seen at the GM displayes at Woodward cruise had huge engines with lots of overlap and they didnt have 02's hooked up. One things consistent, 02's dont read right at low RPM with big cams.




Well I got a call from the guy saying the surging/bucking has gotten much worse.
He said he almost got T-boned making a left hand turn when he punched it and it stalled and bucked, and then took off.

I'm just about all out of ideas at this point. I don't really want to disable closed loop altogether, cuz that's kind of scary. I'd like there to be some fueling feedback just to be safe.

Exactly what is causing the surging? Is it from the o2's seeing fresh air due to the cam overlap?

Any ideas..?

I have already blended the closed TPS timing into the spark timing graph.

I called the guy and had him watch his innovate wideband. he said he was getting a reading of like 16-17 in closed loop instead of it's normal 15.2 reading, so I now know something has changed and I'm thinking it may be fuel pressure. He has an inline (booster) fuel pump thats been making alot of "revving" noises lately... it may be causing a blockage. He also has 39# injectors and an Aeromotive fuel pressure regulator which we have adjusted up to 58psi giving a new flow of a bit over 45#/min.

This is a slight relief since the tune was working great except for the small surge at cruise and slight acceleration.

Here's the most recent datalog. Note that baro is ~86kpa and he's boosting about 2.5psi, and the IAC is maxed out while driving for some reason?

http://www.mediafire.com/?tmmgdtynd3n

Need to open the blades a bit so the iac doesnt max out, the iac maxes out because of the 'throttle follower' function, basicly it just adds some air to the system when coasting to help catch the idle when slowing down or coasting.

Idle air temp looks pretty high to me. Any particular reason why its up around 100F and above.

Still looks like the low throttle timing needs some work. The time he hits the gas and it nearly stalled the timing fell right out of it to nothing during the whole time it was struggling to run. Might want to fatten up the open loop target ratio for the fully warmed up 80C as the issue with the stalling seems to have been in open loop when he touches the gas it nearly goes into a lean stall along with the timing, hard to tell which one is the root cause. Work on both and save time imo.

Wot section is looking scary lean, also appears to have a ton of timing. No knock retard though, is it still enabled?

Narrow band o2's should work fine my fuel trims are dead on at idle and low speed after some tweaking and my cam is pretty big.

IAT's suck cuz its supercharged with headers. Lots of heat under the hood and therefore in the aluminum intake pipes. No intercooler.

I believe I had right around 12.9 commanded in the two "% change to AFR in WOT" tables. His fuel pressure may be dropping at high fuel demand. We're going to hook the fuel pressure gauge to it again and monitor it while driving.

It's a fully built forged 383 stroker with Dart heads (about 9.7:1 or something I think)

The car was in some sort of limp mode when I first scanned it. ESC (knock sensor) circuit failure code. It was holding a constant 5 degrees of KR no matter what. I unchecked that code in the Switch table so it won't throw the code and the 5degrees KR went away. I don't know why the code threw in the first place but the knock sensor still appears to be working as normal (pulling timing here and there under different conditions, but I can't say for sure). Still need to find out why that code threw and fix it before I move to power tuning after I finish divability.

Does it even matter if the IAC maxes out when off-idle?

IAT's suck cuz its supercharged with headers. Lots of heat under the hood and therefore in the aluminum intake pipes. No intercooler.

I believe I had right around 12.9 commanded in the two "% change to AFR in WOT" tables. His fuel pressure may be dropping at high fuel demand. We're going to hook the fuel pressure gauge to it again and monitor it while driving.

It's a fully built forged 383 stroker with Dart heads (about 9.7:1 or something I think)

The car was in some sort of limp mode when I first scanned it. ESC (knock sensor) circuit failure code. It was holding a constant 5 degrees of KR no matter what. I unchecked that code in the Switch table so it won't throw the code and the 5degrees KR went away. I don't know why the code threw in the first place but the knock sensor still appears to be working as normal (pulling timing here and there under different conditions, but I can't say for sure). Still need to find out why that code threw and fix it before I move to power tuning after I finish divability.

Does it even matter if the IAC maxes out when off-idle?


IAC counts are suppose to max when moving this keeps the RPM up there's a table for it somewhere if you want to lower it or if you think the RPM is too high when moving.

IAT are only high cause you just started it, that's kinda normal from absorbing all the heat.

You could try the open loop test as mentioned earlier and also try unplugging the MAF see if you get it to run good off just the MAP. U might want to use VEMaster a few times, u could use this log for that see where it changes your VE tables, seems like it's adding fuel all over the place?

Voltage seems extra low at least quite a bit lower than mine.

Also, since everything seems overall lean you could try lowering the injector size some to richen things up everywhere but the 02's will eventually even that out back to how it is now so your best bet to diagnose this is run open loop until you know it runs good then turn on closed loop and see what happens.


So, first thing to do:
1) Unplug 02's and MAF and run it around on only the MAP. If it runs bad then run VE master until it runs good. Once it runs good compare this to a day or two of 02's. If your MAP readings are always low then maybe it's your location?
2) If it begins running bad again in closed loop then go back to open loop and reset the PCM then plug the MAF back in and see how that reacts.

Where is the MAF and how old is it?

It's a stock MAF with high flow end caps. it's located before the supercharger. It ran horrible with the stock MAF calibration tables. It's running tables that PCM4less gave for his other granitelli MAF or something (which isn't installed). Which makes no sense cuz the granitelli MAF's are calibrated to work with the stock tuning, or so I thought.

I ran it in speed density mode and it took like 8 drive cycles to finally dial in, it kept maxing out the adjustments. It lowered the VE tables over 20 in some spots.
The small surge seemed a bit worse than in MAF mode.

It's a stock MAF with high flow end caps. it's located before the supercharger. It ran horrible with the stock MAF calibration tables. It's running tables that PCM4less gave for his other granitelli MAF or something (which isn't installed). Which makes no sense cuz the granitelli MAF's are calibrated to work with the stock tuning, or so I thought.

I ran it in speed density mode and it took like 8 drive cycles to finally dial in, it kept maxing out the adjustments. It lowered the VE tables over 20 in some spots.
The small surge seemed a bit worse than in MAF mode.


Was all this done in closed loop? Im still trying to see what happens with proper tuned VE's with no MAF and no 02's since they are likely to mess everything up especially with all the mods you have.

Yeah this was all done in closed loop.
Kept adjusting the VE's with VEmasters until the long term fuel trims were dialed in.

Not my car, it's a friends.

Fix your fuel pressure issues first, otherwise you might as well try and ice skate up hill. Your injectors have got to flow what you tell the computer otherwise when you stand on it you will be lean. I am unsure how the stock fuel pressure regulator handles boost. But you need to be aware of the fact that fuel pressure is adjusted according to manifold vacuum to maintain a specific pressure differential across the injector or the flow rating of the injector will creep away from nominal.

His map readings are low because I assume he is at high altitude, and when under his stated couple psig of boost he does get up to 1 bar and beyond.

I would ditch the high flow maf ends,go back to stock, and put the maf in the boosted side if it was me.

Once you have a working solution for the maf calibration, try flipping it over into open loop mode and command around 14 as the target for cruising and richer up high. Log his wideband 0-5v output through the cars a/c pressure port and view it in datamaster custom data. If it is not what you commanded for one airflow value then you have an issue with your maf calibration, if its wrong for all values by about the same % then you have an issue with fuel pressure or injectors not flowing what you think they do.

Never just go in commanding a 12.9 air fuel ratio for wot right off the bat unless you are dead certain the maf or ve tables are right on. Start fat and lean it out so you dont burn the thing down the first time you step on it. It will tell you what it likes when you lean it out some after that if you get it on the dyno or at the track, and then you can find tune it.

some good tuning talk in this post. really enjoy reading and learning this type of stuff.

Yeah he's coming over today and we're gunna put the fuel pressure gauge on his car and see whats going on. He has an Aeromotive adjustable regulator.

Yes atmospheric pressure here is about 86kpa, and for now he's boosting it to about 103-104kpa.

I think the tune was dialed in fairly well. Before it goes into closed loop when u start it, it commands 14.3:1 AFR, and that's what his wideband was reading.