You can make some gross changes to the CTS that will change how the ECM reacts to it. Unplugging it will make the ECM think it's very cold (very high resistance). Shorting the connector's yellow wire to ground will tell the ECM that the temperature is very high (the fan should come on).

 

Hello Ray,

i have exact the same error with my 1988 c4. I have replaced the same parts like you, checked fuel pressure, injectors, etc.
First time i had this error in 2009, i replaced the IAC, the error was gone. The same i had in 2010, with a new IAC the error was gone.
I got this error again one month ago, i replaced the IAC twice but the error is still present. Datamaster shows, the engine runs with lean mixture, maybe the same to you? I hope my english is understandable.

Best Regards from Germany,
Frank

 

Hi..and thanks for your reply.

Strangely enough...my tune started to run lean all of a sudden..then the shiitty cold starts appeared. I altered the MAF tables slightly in the chip to fix the lean mixture a bit. Before all this i was running + - 4 128 BLM with a PCM4Less tune...then the numbers went up to around 136 on average.

Not sure what all this means..i thought i had unmetered air entering after the MAF sensor to lean out the engine? Not sure about that one though.

***QUESTION...If i measured manifold vacuum at 19inches at idle with a mighty Vac handheld...does this mean there are no vacuum leaks??

I swapped out my IAT Sensor under the plenum...actually installed a new IAT Sensor relocate kit i had kicking around in the air duct. I thought i had the issue fixed for a few days but problem is back again...although not as bad? It was really warm for the few days it was starting good in the mornings.

Back to square one...maybe i will pick up a new IAC Motor and swap it out?

 

I think so. What does the IAC do? In my case, the IAC is always wide open, i think more than it would be good for the IAC.
Open IAC -> the ECM want´s more air. But in this case the engine should runs to rich, not lean. My LTerm Counts show the lean running engine, sometimes around 144. In my opinion this makes no sence and i´m a little bit

I would try a other IAC.


I´m not sure. Maybe there are leaks in the exaust system, around the oxygen sensor?

 

Oxygenated fuel will bump your BLMs up a small amount because the air/fuel ratio is different.

A small vacuum leak could do it. Also O2 sensor getting old. i don't know if an exhaust manifold leak would do that. Mine sometimes get loose at the heads (it only happens on cylinders #7 and #8).

To thefbi: how do you know the IAC is wide open? I don't know what "wide open" means as far as the firmware goes. Theoretically it could go to 255 counts, but I'm pretty sure the ECM limits how far it will open to something less than about 150 or so. Anything that high tells me that the throttle plates are closing too much (need a minimum idle adjustment).

 

Hello Cliff,

in my case, the IAC is around 45 when the engine is hot (should be around 20). When the engine is cold, closed loop, the IAC is 145 which is the maximum open rate in a c4. My next step to solve is problem is a minimum idle adjustment.

Best Regards,
Frank

 

I don't think 45 is high. Mine runs around 50, which I consider to be perfect. Leaves room for the ECM to adjust it as needed. I saw a post one time of a guy who was trying to get his at zero. That leaves ZERO room for the ECM to adjust it on the down side.

You said "should be around 20". I have never seen anything anywhere from GM that specifies exactly what is the "perfect" IAC count. From what I've seen it's opinions, guesses and "authoritarian" statements about what it "should" be. I haven't seen any test results or anything that proves that these opinions are valid.

I have seen people say that the IAC is a "vacuum leak", as if that is somehow bad. I don't care what it is as long as the correct amount of air gets into the engine. The way I look at it, the IAC serves the same function as drilling holes in the butterflies of a carburetor to get a better idle. It's very difficult to get a consistent fit between the butterflies and the bores of a carburetor or throttle body. It varies a little bit every time the throttle valve opens and closes.

The IAC also acts like a dashpot on a carburetor. It opens as you open the throttle ("throttle follower" mode) so that the engine won't die when you take your foot off the gas pedal at 45 MPH, slamming the throttle valve shut and virtually stopping the air flow through the bores.

 

Have you tried just slightly cracking the throttle while cranking? That would give you an idea if the IAC starting position is correct. Possibly the initial position vs. temperature is wrong if a custom tune or modded engine?

 

I did the base idle TPS adjustment again last night...no difference in starting this morning...still jumpy and sputtering after it catches for about 3 minutes till warmed up.

I will try cracking the blades slightly tonight while cranking and let you know if that helps or not.

Im still unsure exactly what this test means in relation to any vacuum leaks....

***QUESTION...If i measured manifold vacuum at 19inches at idle with a mighty Vac handheld...does this mean there are no vacuum leaks??

Also...does a slightly sticking EGR Valve play any part on cold starts?
I would like to know how to test the EGR valve with this hand mighty vac??

Ray

 

FYI..i gave it a little throttle while cranking tonight after work and it fired better than usual plus the fact that i revved it after it started and it did not sputter nearly as much as usual?

The car only sat for 9 hrs and it was a nice warm day...might have had something to do with it.
I will give it the same treatment in the morning and see.

I do not have the IAC start up position / temperature in my $32 bin

 

Try a different tuning program then in TunerCat it is IAC startup park position and while I have not looked at the table it is there and would assume temperature related.

 

I am reluctant to change anything in the tune because the problem did not exist until recently...was starting fine and running strong with the custom tune for months before this.

Runs great in closed loop...datamaster scan not showing me anything unusual.....you wanna have a look at the scan??

FYI...Car sat overnight...gave it a little throttle this morning while cranking...it crapped out again when it fired up...sputtering and stuttering for about 3 mins until warm.

I think i will try a new IAC...Can get a new Delphi for $55 to replace my $25 ebay chinese model,,,at very least its a definite upgrade.

 

I measured my vacuum at idle with my trusty rusty Mityvac. I measured at the rearmost port on the passenger side. I saw 19 inches of vacuum at startup, which dropped to 15 when the idle came down.

In my '86 BUA firmware the startup IAC count is set to 144. That is "fast idle" mode, like back in the carburetor days. The ECM reduces that 144 count depending on coolant temperature, faster if the temperature is high at startup.

The main problem with being in open loop mode is that the ECM has no feedback and has no way of knowing if the air/fuel mixture is correct. It reads the TPS and MAF and looks up values in the various tables (based on coolant temperature) and calculates values for injector pulse width and spark advance. [EDIT:] I just thought of another one while reviewing my post. The ECM also looks at the battery voltage and will adjust the injector pulse width if the voltage is too high or too low, so that value is important also.

The ECM uses the values in the BLM table as the basis to determine injector pulse width while in open loop mode. Mine starts in BLM cell 5 because of the high idle speed and then drops into cell 0 when the idle comes down. What BLM cells are you in during warmup, what BLM values do you see and what are the injector pulse widths? In my firmware the minimum injector pulse width is set to 1.69 milliseconds (you can't get less than that). You can hit the minimum limit if your injectors are too large. Your 24 lb. injectors should be OK there.

The IAC controls the idle speed by adjusting the air flow (assuming the mechanical part of the minimum idle speed adjustment is OK -- actually it looks at the RPM also and will try to compensate even if the throttle blades are incorrectly set). What IAC counts do you see during warmup?

The IAC count needs to be high enough at idle when the engine is warm to be able to control idle if the mechanical minimum idle adjustment is too high. I think it ought to be around 50 (my opinion only -- see my post #27). If it's too low the ECM won't be able to lower the idle if there's a problem (such as binding in the linkage or throttle shaft -- a problem with the cruise control cable can also cause a high idle (happened to me)).

 

Then you have to assume something has failed as the ECU did not just get stupid one day if no one changed anything. I had assumed the tune not finished so must have read in something not written.there are really on 4 or 5 sensors that can cause the problem. O2 is out of the picture cold. If you check air temp and water temp and TPS and they read realistic they are out of the picture as well. I don't know if you have a MAP sensor or not if so compare that to a vacuum gage on the intake manifold and see if that reads properly if so out of the picture. It kind of leaves you with Mass air sensor or cold start injector if you have one. Your right foot is just as good at idle speed control as the IAC so i doubt it would be that.
Dave

 

Cliff / Dave...Thanks for following this thread so closely and offering your advice.

I tried the new IAC Motor this morning... no change, but at least i know i have a quality Delphi part in place now.

I have uploaded a couple of my latest Datamaster scans if you guys are inclined to have a look at things and maybe see something out of the ordinary.

The first one "Friday2" is last week before i started to change anything

http://www.mediafire.com/?g3r1dl2pvveb9ph

**************************************** *************
2nd one is yesterday after i did a new idle rest procedure and relocated IAT to air box with new sensor and harness, adjusted MAF tables to fix BLM. Open loop on this one starts out with a hot motor so dont worry about the temp reading on the CTS...it is correct

http://www.mediafire.com/?u3hb11fay6p53gy

I would appreciate your thoughts as im still a novice at interpreting these scans.

Ray

 

Anyone have a chance to look over the datalogs?

My neighbor the wannabe mechanic mentioned to me that something could be fouling 1 or more spark plugs during start-up...coolant or fuel leaking inside.
Im sure it is not fuel after taking all those fuel pressure readings that seemed right on.

Coolant leaking inside?? is it feasible? How do i check that out?
im not low on coolant

 

The datalogs look pretty good generally. The only thing I saw that looked strange was that the TPS voltage was jumping around.

You can see if you've got coolant leaking into your cylinders by removing all the spark plugs and cranking the engine. If there is coolant in the cylinders it will get blown out and you'll see drops on your exhaust manifolds.

 

Thanks for looking at the scans. I didnt think the TPS readings jumped too much or often...primarily with engine RPM...Do you think there is an issue there...its a new part...i know that doesn't mean jack..

Key on with datamaster and moving the throttle by hand...TPS seems to increase voltage very smooth up to 4.5v...would suggest there are no "flat spots"

With regards to coolant in the cylinders. Im not convinced we are looking in the right direction. My neighbor and i started the car tonight with the usual long crank and sputter start...there was zero white smoke if fact there was no smoke whatsoever associated with this start-up and even as it idled for 2 mins...zero smoke. Plus the fact that im not losing any coolant..i can pretty much rule that out.

Also..the plugs i pulled a few weeks ago while this issue was going on are all pretty much burned with the normal tan color...no oil..no glazing..and none that have been coolant washed.

not sure where to go from here...fuel filter is a year or so old...might be worth a shot? but then again i get proper pressure at the rails with key-on...who knows,,,not me?

 

Any funny spots in the TPS will show up on the low end because that's where you spend most of the time. The wiper on the TPS resistive element bounces around at idle from the vibration of the engine and pounds on the element in that one little spot. It's pretty robust but can wear out over time.

I don't remember the TPS voltage moving much (if any) at idle on my car. I don't have any scans lying around to look at so I'll have to hook up to the car and see what happens.

 

I have decided that my problem with cold starts is the fuel pump. I have to take off the air coupler then spray starter fluid in the TB. It fires right up and until it sits over night will start when I turn the key. I am wondering how to tell if the fuel sender needs to be replaced at the same time ? i just ordered a fuel pump and in tank filter with a new in tank fuel line a short hose. It is all coming in a kit. I just hate to have all this off the car and find out the fuel sender should also be replaced. I know the gasket will have to be replaced after taking the fuel sender off the tank.