WinALDL Data Log Need Help -- (Another truck)

[jeff in co]

Well after several attempts to get my home made serial cable to work with "any" computer I had access to, I finally started seeing the magical numbers dropping in on the table!

Used secondary laptop with XP and used the docking station (that had the serial connection). Couldn't get it to work off of laptop battery while docked so used my DC to AC converter, plugged the dock in with power, dropped the laptop and away it went! Sweet!

Now, I need to figure out what I'm looking at.....

I am trying to find my high idle problem each time I place the truck into park. Warm or cold, it still idles around 1100 to 1150 rpms. Can anyone interpret this data log to decipher if I have a bad sensor or ????

Secondly, I do have a few "very light" sputters while holding the throttle. In this run, I held it around 1800 rpms and noticed the engine every once in a while, a few bobbles in the engine tone.

Any help would be greatly appreciated!

 

[jeff in co]

Here is one more data log that I did yesterday with the engine warm already. Pretty short run but shows the high idle pretty good.

Don't know if the rich/lean numbers have anything to do with it but would love some input figuring this out.

 

[Russell]

Hey Jeff,

Your sensors look pretty normal in your data logs. While the engine doesn't appear to be idling overly high in your logs, 7-800 rpm, it is running very lean. The noise you are hearing is a lean pop in the intake. If your engine is truly idling high, then I would be looking for a vaccum leak somewhere on the engine. If the idle is normal like the logs say, and your tach is just out, I might suspect that your fuel pressure may be a tad on the low side.

Let us know how your diagnostics go!

 

[jeff in co]

Thanks Russ,

I've checked as much as I can on a handful of parts.

- replaced almost all of the vacuum lines
- replaced the TBI to intake gasket
- replaced the IAC
- replaced the injectors
- tested the MAP sensor, replaced with a used one and tested again (same readings)
- Chevy dealer replaced the entire distributor however it does look like the same cap (and assumed rotor)
- New EGR Valve
- New PCV valve
- Replaced Cruise Control Plunger and now cruise works

Sometimes it does get down to 750 or 800 but that is after driving around and then placed into park. Where I really notice the high idle is when I start it back up after warm. It just seems to hover around 1100 rpms (both verified with dash tach and aldl program). When I shift from park to drive, I really notice it because I feel a little truck movement due to the engine running a little high.

The second zip file shows this more specifically. Started in park, waited a few minutes and then drove down to a stoplight and stopped data logging. Right where the idle drops to 700-750 is when I place it into "drive".

I am really shooting in the dark on this truck. From carfax, it has 314,000 miles on it but the block is blue. So I'm guessing it was rebuilt at some point. I just don't know how much was done. I am nervious about the fuel pump going out (strickly from the mileage viewpoint). The fuel gage is a little slow on accuracy.

Any other ideas? Appreciate the input.

 

[Russell]

Don't suppose you thought to check your vacuum booster? Lots of people forget that it has a diaphragm that can fail and cause a pretty huge vacuum leak. One way to check it is to pull the hose off it after the truck has sat for a few minutes. It should still have a bunch of vacuum on it when you pull the hose and air should rush in. Make sure the check valve is good too.

Also, don't count out the possibility of a leaking intake gasket, or around the throttle shafts in the throttle body.

All that said, the fact that your IAC is sitting at around 25 counts where it belongs is kind of strange for a vacuum leak situation. There is a good possibility that the ECM would like to close the IAC more, but it can't due to either a sticky IAC, or some crud in it's passageway preventing it from closing any further. The IAC reading is not what the ECM want's the IAC to be, but rather what position it is actually in, btw.

You can check it by pulling the IAC out (round canister on the pass side of the throttle body with the square 4 pin connector) and working the worm gear in and out while lubricating it with a dry white lithium grease. Then take a look at the condition of the pintle, and also the port that it seats against inside the throttle body. Clean up any crud that you find on either, and make sure that the passageway into the throttle body is clean and clear.

If you pull the IAC out and move the position of the pintle by working the worm gear in and out, you will need to re-set minimum air so the ECM can re-learn the pintle position. To do that, run the engine until it is at full operating temperature. Shut down the engine then jumper the ALDL port so that the ECM starts to display it's diagnostics codes when you turn the key on. This also causes the IAC to close entirely. Leave the key on for 20 - 30 seconds without starting the engine, then un-plug the IAC. Turn the key off, and remove the ALDL jumper. Try to start your engine. If it will not start, you will need to bump the idle up a bit with the screw on the driver's side of the throttle body. There is a possibility that it may be covered with a thin metal plug, which you can just knock out with a screwdriver. After the engine is started, lower the idle with the screw until the engine is idling as low as it can smoothly, typically around 350 - 400 rpm. Shut the engine back down, and reconnect the IAC. If your engine has set a code when you start it back up due the IAC being disconnected, you can re-set it by un plugging the battery for 10 minutes or so.

Again, there is still that possibility of your fuel pressure being a hair low, you can purchase a pressure gauge and adapter that installs between the inlet fuel line and the injector pod to help you determine if you have enough fuel pressure. They are not too terribly expensive, under $100 bucks, and are super useful for diagnostics.

BTW, you mention that the engine is not the factory original -- Has your engine always behaved this way, or is this something that has cropped up since you bought it?

 

[jeff in co]

Thanks Russ for the tips. I'll go through the steps on the IAC unit. I did replace this piece a month ago but has always been a little suspect. Before installing the new unit, I did compare it to the existing IAC and noticed that the new one was not as long as the old one. I don't know if I needed to pull it out (or if it would extend any more on it's own) but did slap it in.

I'll try taking it out and seeing if I can extend it out any more. If not, maybe I'll go back to pep boys and get another one.

This was my old truck 15 years ago and I bought it back a few months ago. I have been working on it quite a bit and it's been neglected mainly from PO funds (I suspect). A lot of little things but nothing too bad. Just items that may not be at the top of the list for replacement if funds were tight.

It originally would not start at random but we later found out a new distributor was needed. Since then, it has never failed to start. I wish I knew more about the engine but can only assume they re-built it at some point. Blue color on the block, no smoke from exhaust and has 314,000 miles (according to carfax). I don't know when.

I'll let you know how the IAC thing goes. Thanks again!

 

[Russell]

Ah, that makes sense then! If you just popped a new IAC in without re-setting minumum air, the ECM will have thought that the pintle was in a different location than the other one, aka 25 counts on the original would not be 25 counts on the new one.

Before you take anything apart, just run through the minimum air procedure and see if that helps you out any.

 

[jeff in co]

Ah, that makes sense then! If you just popped a new IAC in without re-setting minumum air, the ECM will have thought that the pintle was in a different location than the other one, aka 25 counts on the original would not be 25 counts on the new one.

Before you take anything apart, just run through the minimum air procedure and see if that helps you out any.

I did do a reset several times when I originally installed it. When I did the disconnect, the engine actually ran really nice and the rpms stayed down where they needed to be. When I originally did this procedure, I did hear the IAC valve making some clicking noises so I assume it was trying to fully extend.

Just spent a few hours driving to Chevy dealers to locate the plastic air diverted piece for my heater blend door, so I will now move onto the IAC valve and let you know how it turns out.

Thanks.

 

[jeff in co]

OK, I pulled the IAC valve and was able to pull out the pin just a little. I tried pushing it back and forth but it wasn't, in my opinion, very smooth. However, when I did re-install it, the idle was definitely lower. I went through the reprogramming and after plugging it back in again, the idle remained close to 700 rpms.

Feeling better, I went and drove for about 10 miles and then stopped again. Idle was close to 750 or 800......a nice spot. I then turned the truck off.

I came back about 5 minutes later, restarted, and the idle went back up to about 1100 rpms. It seems to hold the lower rpms until I turn it off and then restart.

Would this indicate a bad IAC valve? The brand name I have is TRW (I believe). Maybe I should just return it and find an AC Delco one.

Shouldn't the pin go in and out smoothly?? Just don't know.

Thanks for any advice.

 

[Russell]

To move the pin, you need to gently rock it back and forth to work it up and down the worm gear, shouldn't be able to just push or pull it. That said, it does kind of sound like a bad IAC, I hate just throwing parts at anything, but you may want to give it a go with an AC-Delco unit from the wreckers. They don't normally go bad, in my experience, and a junkyard piece should be fine.

 

[jeff in co]

To move the pin, you need to gently rock it back and forth to work it up and down the worm gear, shouldn't be able to just push or pull it. That said, it does kind of sound like a bad IAC, I hate just throwing parts at anything, but you may want to give it a go with an AC-Delco unit from the wreckers. They don't normally go bad, in my experience, and a junkyard piece should be fine.

Well, I swung out to the junkyard yesterday and picked up a "new IAC valve" off a 90' pickup. Actually pulled 3 and found the one that looked the nicest!

Anyways, I just slapped it in and drove away. Engine drove nice all around town (as before) and once I stopped and placed in park. Idle was lower (800 rpm or so). Turned it off and then re-started the engine. It immediately went back up to 1,100 rpms and didn't drop.

So, basically the same reaction as the other IAC valve. High idle at startup.

I did not re-set the junkyard IAC valve on the computer. I just place it in and drove away. I will go and re-set it but I'm not too optomistic much will change.

It just seems like a computer control type of thing when it perfect for a long time and then when I turn it off and restart, the problem is there again. Strange.

 

[Russell]

That definitely is unusual behavior... I am not too sure what to tell you man! Almost like your ECM wants to run that high in RPM. Does the ECM idle high even on a full temperature start? If you let it idle for a minute or two, does the idle eventually work it's way down to 750 - 800 rpm?

 

[jeff in co]

That definitely is unusual behavior... I am not too sure what to tell you man! Almost like your ECM wants to run that high in RPM. Does the ECM idle high even on a full temperature start? If you let it idle for a minute or two, does the idle eventually work it's way down to 750 - 800 rpm?

That's the thing, it does it when the truck is at full running temperature. I've let it sit idling and it doesn't seem to go down any.

I guess I can lower the idle a little more but it sure sounds like it's going to stall when I do the IAC reset process. Afterwards, I'll do another run on the data log and see how long the idle stays high if I just let it sit for 5 minutes.

I won't be able to get to it until next week but will post back any results/changes. Gotta go to Idaho tomorrow.....

Thanks for your help Russ....