vacuum and idle question

walter pilie

Registered User
My 92SC is just about complete.

New short block from Ford. Stieg heads and cam, w/1.73 roller rockers, Bosch matched 42# inj, 87mmPro-M MAF, 85mm MP Tb, 3.5 intake tubes and K&N, ARP head studs, ported SC by MP, raised top, 190lph fuel pump, 2.5x3x2.5 exh w/o cats, raised top, MP IC kit with pusher and puller fans, 180F tstat, Magnecor wires, new plugs, mid-length headers, BHJ balancer, underdrive pulley set, 10% SC pulley, 4.2 head gaskets, and chip by APTEN. I'm sure I've forgotten something.

The car runs like a bat out of hell when getting into it. Idle is rough (cam lope very noticeable). If throttle is opened, and returned to idle, it will kill. Idle is at ~750-800rpm, and manifold vacuum on dash gage is about 8"hg at idle (before mods, this was 19"hg). Seems to be rich at idle. We would have expected better vacuum for idle, and idle at 1000rpm. No check engine lights.

We are in touch with Brian at Apten, and are checking basics such as cam sensor position, coil pack to plug wires. Will likely send chip to Brian to adjust the idle and try again.

What I wonder is what others with similar setups have experienced after the cam and heads and intake, etc, etc mods.

Any experiences that anyone might be able to share? Especially any fixes to the idle problem.

I saw a thread that mentioned adjusting the idle screw on the TB and the idle control bleed screw (not sure where that is).

thanks in advance for any advice.
 
Not all TB's have an adjustment screw. Besides the computer will just close off the IAC and you might not be able to add enough air.

You didn't mention cam specs other than rough idle. With a larger cam you will need to add timing and raise the idle to about 900-1000 rpm. 8" is real low I think Vernons cam runs about 12" and it's about 218/224@ .050" Idles at about 1K as well

Make sure Brian knows exactly what cam you have, it sounds like a pretty big one. Whe you raise the idle some of the vacuum will return.

What is the PRO-M curve for? Did you give the correct info on that?

Jeff
 
The block, cam, heads were bought through Brian. He has all the info on that and all components. TB, MAF, tubes, temps, injectors, etc., etc.

I think that IAB may be not responding like it should, perhaps. It would not catch up quickly before all the mods, when an accessory (e.g., cooling fan) went on. But we debate over whether it could be the root cause.

Was wondering if there other adjustments to be wary of.

Thanks for the reply.
 
I am having the same problems, and I have attempted fixes of all sorts. Now I just live with the problem, my rps dip really low when I come to a stop, but rarely ever dies. Brian also burned my chip and set the idle at 950. This is where my car idle bounces to after almost dying at each stop or when I push in my clutch. I have many of the same mods, but not as big on the intake side.

I have replaced the speed sensor and wiring. Ive cleaned the maf and checked for vac leaks. I used to unhook my battery because the car would run good (high idle) until the computer relearns to nearly die at a stop.

Symptoms: High idle at start up. Runs good cold, but when it warms up the low idle with the clutch in begins.

Temporary fixes for me: slow down for a stop light while in third gear just as the idle gets to 1000 and the car starts to buck, I push in the clutch and then the rpms dont drop. Also driving with the A/C or defrost on bumps the idle upto 1500 when I push in the clutch. Then it drops to the chip setting of 950.

I know you are looking for a permanent fix and so am I. My IAC valve is two years old, my tps is original. I am getting phantom codes for my O2 sensors reading rich/lean, but I replaced those during the rebuild 7-03 and again after getting these codes, 11-03 ( under warantee) I am leaning towards my IAC or TPS, but I am not getting the "usual" symptoms or codes for these being bad.

My cam was installed at 110 degrees. the duration @ .50 is 240/240 my lobe lift is.3210 and seperation is 114.0.

My vaccum is also at about 8.

What year and transmission do you have Walter?

Brandon
 
reply to Brandon,

mine's a 92, with 5 spd, and stock rear end. I had the entire suspension done over a year ago. Had it lowered by 1-1/2", and all new bushings, oversized sway bars, and the front and rear poly bushing set, and solid tranny mount, and differential mount replaced. Tokico's replaced a year before. Rebuilt halfshafts, heavy duty u-joints, new clutch (Ford) with throwout bearing, hydraulic slave unit, and pressure plate replaced. And.. oh yeah, pilot bearing and seals on tranny tailshaft to stop nasty leaks. Changed the front spindles to '95 units to allow replacement with PBR dual caliper piston brakes, with cross drilled rotors. While at it, all new wheel bearings, hoses, and on and on.. Had to drop the tank because of bad sending unit with a cracked weld at the flange which leaked fuel, and put a 190lph at that time.

Once all this nonsense gets over with, I will go to a psychiatrist for treatment, as the tab over the past 4 years will be serious enough to contend with the purchase of a new Cobra (w/SC). I'm never sure whether to call this whole progression of fooling with this car an addiction or an affliction! But...can't turn back now!

This is my 2nd SC. I had one of the first '89's to be shipped to New Orleans that year (new and a dream car, and 100% stock til the day I sold it). I wish many times I had that car to be doing all this stuff to. It was a better car than my current '92 (which isn't bad, and was garage kept all its life). I'm "restoring" the car, kinda. The exterior and interior will be 100% stock, except for custom floormats, a 140mph speedo, and the ripper shifter (which nobody but me will notice). All the goodies are under the car and the hood. A sleeper. But now, the cam is going to give it away! I hope some of the lopiness calms a bit with the idle set higher.

We are not complete with the car yet. Watching the engine as it idles and learns. Settings not yet final. Has not CEL'd, and no codes showing up so far. But only very short test drive done by my mechanic, as I have not had time to get to the shop this week.

Will be happy to share whatever we discover with you. By the way, did you have any dyno test run done, and send the info to Brian? I wonder if that helps him fine tune the chip to get the fuel-air better. I wonder if that would help with your O2 sensor problem. Brian suggested doing that before we ever started on this upgrade. After all the reading I have done on the Ford injection system, I still don't get totally warm and fuzzy with the tuning. I have thought about making a run to St. Louis to have Brian do additional tuning, once we get it as good as we can and trus driveability.

Best part is, I am not a drag racer! I do, however, like running the car on some backwoods roads, and I think that I will get the kick I wanted, but it still has to be driveable and reliable. I hope I haven't stepped in it!:p
 
You should have done a dyno run for sure. You are not going to get it right the first time and you shouldn't expect it either. It might take 1 time it might take 10 times. You do need to dyno it and send the info to Brian.

Brandon,

Your problem sounds like a bad IAC to me, it's probably gummed up. And are you sure you are using a single pattern cam with that much duration? At 240@.050 I am suprised you don't have to set the idle to 2K just to keep it running. If you are running that much duration, what is your CR? With a low CR the engine is going to barley idle and the will have lower vacuum than one with higher CR.

Jeff
 
This is a Comp cam, suggested to me and bought through Steg. I know when Brian burned the chip we didn't really know at the time which Cam I had. I also didn't give him any dyno results because I didn't run the car due to the fresh rebuild.

I am going to try and clear up my idle issue and then get to a dyno or even go to STL. to see Brian ( I am in Kansas city).

Jeff, Thanks for the tip. Do you think it is the IAC because of the symptoms I describe? Could it be due to the cam grind? I know when I requested the cam I didn't want the most agressive grind, just something streetable. BTW what is CR? What info can you use off of my cam card to help me understand which grind I got?

I will post my cam card ASAP, my digital camera battery is dead.

Thanks
 
CR is compression ratio. The duration at .050" is important a cam with 240* duration is big block territory. If you stick the same cam in engines with different displacments (all things equal) you will see that the RPM band of the engine will shift higher as displacment goes down. Here are some displacment per cylinder figures.

454=56.75 cu in
350=43.75 cu in
231=38.50 cu in

a 240 cam in a 454 might not have any trouble idling at about 7-800rpm but in a med sized V6 it would be like a pro stock car.

Chances are if you requested a mild grind you have a cam around 208* duration and about .520 lift cam


I think it is the IAC because of the problems you have during sudden changes in throttle (blade) position. When the IAC is sticky it won't respond as quick causing what you are seeing. Without being there it's hard to say, but that is what it sounds like to me.

Jeff
 
I am planning on cleaning the tb and Iac this week. I know the IAC shouldn't be cleaned because the diaphram is brittle. But my Ford Iac is only two years old so I will clean it before replacing.

Jeff, I hope that you are right about my cam grind. But the card seems to read 240 intake and exhaust. I will try to post those pics of the cam card tomarrow. I suppose if I am reading this card right and the grind is that aggressive, that the car wouldn't be able to drive at all.

Walter, good luck with your SC. Sorry to take over your post.

Thanks
 
I hope we can see this I dont have a scanner..........
 

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I had this same problem with my 5 speed dying. It's the curve in the EEC for the IAC and 'desired idle airflow'. When you change MAF's it F's with the curve values.

For instance, the stock EEC idle airflow curve is only like 3 points. The idle in neutral has one at 704rpm (about 20 kg/hr) one at 1248 rpm (around 30 kg/hr) and one at 2000 (around 50kg/hr) rpm. With a stock MAF, an idle reading is around .8v which equates to 20.28 kg/hr in the MAF function. Works perfect for an idle of 700-750 rpm.

Now, change to a high flow MAF. In my case a 76 C&L that has had a 36-38# tube and now a 50# tube. With the 36# tube, my idle MAF reading was just under .7V at 800 rpm. With the 50# tube, my idle MAF reading is about .58V.

.7V on the 36# C&L MAF function is about 20 kg/hr, so the desired airflow curve thinks it should be at 700 rpm. With the 50# tube, .58V equals about 22-23 kg/hr in the MAF function. Which also screws with the EEC. The desired airflow table has to be adjusted to match the MAF function or the EEC will either idle high or low all the time.

Also, there's a setting in the EEC for 'Dashpot Decrement Rate' and 'Dashpot Clip'. That adjusts the rate the IAC opens/closes and the airflow it will allow at idle. The stock functions only have a few points and don't appear to work well with mod'd motors. I've had to tweak mine to allow the IAC to open more and change at a slower rate.

Good luck,
Steve
 
Steve, Was this a reply to my idle at a stop problem, I know this is walters thread. Is the adjustment something my chip burner (Brian at Apten) can do. Is this something he may have already done? What info should I relay to him to suggest that this needs to be added to my file? He has already bumped my idle to 950rpms, are these adjustments you suggest different? Thanks for the input, this seems to be exactly what I am looking for. What should I ask my chip burner to do?

THANKS!!!
 
Brandon, I agree that Steve's info seems dead on target to contend with the problem that you (and now I) have experienced.

I am hoping that Brian's fixes to my chip will take this into consideration, but if not, I will talk to him about it. I know he is adjusting the settings to achieve 950RPM at idle (I would be good with 1000, give the underdrive pulley set I have). I have not been able to speak to him lately, he seems to be out the shop last couple of days.

I believe the dashpot setting corrections are exactly what is needed to prevent the car from stopping after returning to idle from cruising. That's the analog to an old anti-stall dashpot on a carburetor, and I recall the exact type of problem on carburetors when the dashpot failed - change the dashpot, problem solved. So, change the rates of signal change on the EEC to control the IAC, and you ought to achieve the desired results. I am changing the IAC on mine, as it was not fully responsive before the enginen swap, and the shop just hadn't remembered to change it. I suspect a new Ford unit will help, as opposed to a 12 yr old one with 100K miles use that's now on there.

Steve, what device are you using to perform your EEC tunes? This might be just what the doctor ordered. I have personally been leary of attempting tweaks to a/f, timing, etc. because of experience, and hence have not seriously looked into tuners that you can plug into a laptop. But, I see a lot of value in them for trouble shooting. I would appreciate feedback on pro's/con's of available units. I wouldn't know which unit is best, or useless for that matter, but would buy one (if not cost prohibitive) if it was a useful tool to handle most diagnostics and setting issues with SC's. Thanks in advance.

Thanks for all the input, and by the way, guys, I'm happy to have you "take over the thread". This is a great forum and maybe my problem gets solved with a little peripheral learning thrown in, as well.
 
Glad my post helped shed some light for you guys.

I'm using an EEC Tuner right now. Although, I'm 'on the list' to get a Tweecer when they're available. I'm also thinking about getting a wide band kit I recently saw in Car Craft that can datalog AF readings. I think it was $450 and that would be a huge help to have one in the car for drivability tuning and not just WOT on the dyno.

I'm sure Brian knows about these settings. It's just something that will be specific on each car/MAF/airbox setup. You almost need to take a voltage reading from the MAF at idle to get it right. Just changing the sampling tube in my C&L caused problems for me (went from idling normally around 850-900 to idling at 1100).

The desired airflow setting helped me get the steady state idle right. At first, I couldn't figure out why if I set it for something as low as 700 in the scalar table that it would idle at anything but. It seems that the scalar setting and desired idle airflow values work together to set the idle.

The dahpot decrement rate got it to not stall when pressing in the clutch. It was driving me crazy that my $8k worth of engine work and bolts on made the f'ing thing stall when pushing in the clutch at 25mph in 3rd.

Here's the stock tables for a typical 91-93 EEC (the W1M that I have now). If you notice, there are several '0' settings that can be used to make a smoother curve. Knowing your specific MAF function and idle MAF voltage are very important to getting it right and each setup will be different. Even your throttle plate setting can make a difference. If the throttle blade is open even a few degrees more at idle it will affect the amount of airflow you need from the IAC. I think my throttle blade is open a bit more than stock and/or the 75mm TB flows a bit more for a given closed setting. I believe (my current file is on my laptop sitting a work) that I had to lower the airflow from the IAC to get the idle down, but slowed the rate of change.

DESIRED_IDLE_AIRFLOW_IN_DRIVE # Desired idle airflow in drive (Kg/Hr)
( 4080, 48.903 ) ( 1008, 48.903 ) ( 800, 29.3418 )
( 608, 19.5612 ) ( 0, 0 ) ( 0, 0 ) ( 0, 0 ) ( 0, 0 ) ( 0, 0 )


DESIRED_IDLE_AIRFLOW_IN_NEUTRAL # Desired idle airflow in neutral (Kg/Hr)
( 4080, 50.604 ) ( 2000, 50.604 ) ( 1248, 31.2554 )
( 704, 19.7738 ) ( 0, 0 ) ( 0, 0 ) ( 0, 0 ) ( 0, 0 ) ( 0, 0 )


DASHPOT_CLIP # ISC air at closed throttle. (Kg/Hr)
( 16383.8, 3.26906 ( 800, 3.26906 ) ( 300, 2.17937 ) ( 0, 1.08969 ) ( 0, 1.08969 )


DASHPOT_DECREMENT_RATE # Dashpot decrement step at given flow (Kg/Hr(flow), Kg/Hr(step))
( 435.443, 0.272422) ( 54.4312, 0.272422 ) ( 4.07968, 0.106311 ) ( 0, 0.0265777 ) ( 0, 0.0265777 ) ( 0, .0265777 )
( 0, 0.0265777 )

Good luck guys. I've heard good things of Brian so I'm sure he'll help you out. It just might take a few times because of all the variables.

Regards,
Steve

Ohh.. and Jeff... that's a off comparision of dashpots!!! I thought they were only used on carbs to kick the idle up when the A/C was turned on. Never knew they had other uses. :)
 
Steve, Thanks for all the in depth info, I am sure Brian at Apten can make the right file adjustments to get me going. I felt the same way after all my mods, money and time, and the car wont idle. I did most of the work myself and really second guessed a lot of things when I couldnt get my idle right.

I just have a couple of more questions. I was planning on removing my TB, IAC and TPS to clean them and reseal everything. The IAC is two years old so i hope it isnt already dead. I am getting the phantom O2 codes, and was hoping this would clear things up (until I found out about the dashpot #'s).

#1Could these O2 codes be because of the dashpot settings, or due to an out of adjustment or malfunctioning IAC?

#2 Should I clean or replace the IAC before or after I ask Brian to reburn my chip? Or not at all?

#3 I used Dr. Freds' Idle setting FAQ from the tbsbec (unsuccesfully). Could my Idle screw adjustments be off? Should I change these before or after my reburn? Or not at all? I just need to figure out in what order I should or shouldn't do these steps.

Thanks for everyones input on this. I will be contacting Brian to see what he suggests, and with his permission I will report his findings. So this doesn't end up as one of those unresolved threads that I keep searching through for my solution.


Here is the Idle setting FAQ:
 

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Steve,

would it be possible to retrieve the settings that you have on your laptop? It may be helpful to see how far your changes had to go to achieve results vs. the stock values you provided in your latest post.

thanks
 
idle woes

just an update... 2nd chip sent by Apten made the idle work like a charm. But.... it wouldn't accelerate at all. Spoke to Brian and he said that they had a problem. The 2nd chip was supposed to solve the idle problem but that he was using the 1st chip's data for the driving parameters. In the process, the data from the first chip didn't transfer to the 2nd one. So, he sent a third. This one was supposed to do the trick.

Unfortunately, the 3rd one isn't right either. The idle problem is partially solved. It will idle when started, and it's not so bad, but drive it, and when you let off the accelerator and push the clutch in, it dies unless you keep blipping the throttle.

More tweaks needed.
 
Don't fret he is trying to tune a car that he can't see or touch. It's like trying to open a combination lock that is in a box with 2 holes in it to get your hands in and someone else telling him a number here and there.

I didn't see if you did a dyno run or not.

Good luck, it'll run like stink when it's sorted out.

Jeff
 
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