93 s10 Blazer, error DTC-35, idle air control


dmb359
02-07-2009, 11:30 AM
I finally scanned my blazer, and got DTC-35, Idle Air Control

It has been starting fine, and for a while now, when I am in park, or stopped at a light for a while, the check engine light comes on. I have holes in the exhaust and no cat from previous owner, so it's noisy and does smell like exhaust at times in the cab, so I didn't think much of it.

Now (past week or two) it's been revving quite high when it's in park. I start it up to warm it up since it's winter, and it will idle high until I get in and shift into Drive or Reverse, then the idle drops. When I shift back into park before shutting it off, it revs up then too.


Where should I start?

Chris Stewart
02-07-2009, 01:44 PM
Blue Bowtie has a good explaination and pictures here:

http://www.automotiveforums.com/vbulletin/showthread.php?t=940598

dmb359
02-09-2009, 09:53 AM
thanks, I'll try cleaning it first and see how that goes.

dmb359
02-11-2009, 08:54 AM
cleaned it well, and it didn't change a thing, still idle's extremely high at start up.

Any good cheap place to get a new idle air control valve? or any other ideas?

Chris Stewart
02-11-2009, 07:45 PM
You might check the wires going to the IAC for breaks or shorts first

dmb359
02-12-2009, 06:06 PM
Checked the wiring, doesn't look like any damage, breaks, burning, or anything near the unit, and the plastic wire loom is still around it and in good condition

I put a new idle air control unit on today, and it didn't really change.

Idle was high at start up til I shifted into drive. I took it for a spin, and when I stopped at a light, the CEL would still come on after sitting for like 30 seconds. When I got home I put it in park and left it for a bit. The idle went up like usual, probably around double normal idle (no tac), and it had a sputter to it while idling. I shifted back into reverse, and the idle dropped to normal, then back to park and it did the same thing.

not sure where to go from here.

Chris Stewart
02-12-2009, 07:37 PM
You might check the "starwheel" inside the distributor. The module, with the "stars" tell the computer the engine speed, like a crankshaft position sensor on newer stuff.
If one of the "stars" is cracked or broke, the engine has to depend on the MAP sensor and TPS for engine info.

dmb359
02-15-2009, 12:30 PM
I picked up a filter for the EGR valve, and when it warms up a little outside, I'm going to pull that off and clean it and see what happens. The EGR valve has been replaced at some point (before I bought it), because it looks fairly new, so maybe this issue has happened before, i've heard its a common problem with old blazers.

I'm gonna check around a little for damaged vacuum lines too. Does anyone have a vacuum diagram or anything showing the location of all the vacuum lines?

Bartone_ml
01-28-2010, 11:14 PM
I have this exact same problem on my 95 Tahoe. I also went down the path of changing the IAC with no change. This is starting to get frustrating...

I did not clean the passage ways this evening when I changed the IAC. However, the RPM is about 1500 when in park and neutral. It does drop to ~900-1000 in gear with the foot on the brake.

The interesting part it the RPM's shoot up to 1500 when I first start the truck. Then it works it's way down to ~900. After driving it, I seem to have this issue...

The CEL comes on when sitting at a traffic light. When I drive for a bit, the light goes out.

All thoughts are appreciated.

Mike

blazes9395
01-29-2010, 12:07 AM
I picked up a filter for the EGR valve, and when it warms up a little outside, I'm going to pull that off and clean it and see what happens. The EGR valve has been replaced at some point (before I bought it), because it looks fairly new, so maybe this issue has happened before, i've heard its a common problem with old blazers.

I'm gonna check around a little for damaged vacuum lines too. Does anyone have a vacuum diagram or anything showing the location of all the vacuum lines?

Is this the CPI (W) engine or TBI(Z) engine?

In either case, this applies to both.

The EGR will not cause it to idle high, it will cause it to stuble/stall, and run poorly, but not idle high.

Have you checked to see if there are any codes stored? If there are, you need to get that taken care of first. Do you notice any vacuum leaks? For the z engine, check the throttle body base gasket, for the w engine, the upper intake gasket tends to crack and leak, causing a high idle/lean condition.

Try this, ground out the ECM, like you get the codes, wait 30 seconds and with the key left on, the service engine flashing, get under the hood and unplug the IAC valve. Shut the key off, and unground the service port. At this point try starting it? If it starts and idles high, your IAC needs to be replaced. If it doesn't start, hold the pedal downa little until it starts. If it starts and now idles real low/stalls, you IAC is working.

Hope this helps.

blazes9395
01-29-2010, 12:10 AM
I have this exact same problem on my 95 Tahoe. I also went down the path of changing the IAC with no change. This is starting to get frustrating...

I did not clean the passage ways this evening when I changed the IAC. However, the RPM is about 1500 when in park and neutral. It does drop to ~900-1000 in gear with the foot on the brake.

The interesting part it the RPM's shoot up to 1500 when I first start the truck. Then it works it's way down to ~900. After driving it, I seem to have this issue...

The CEL comes on when sitting at a traffic light. When I drive for a bit, the light goes out.

All thoughts are appreciated.

Mike

As for your Tahoe, you mention your CEL is coming on and then going off, you have to get that checked first, most likley will help you get the problem solved. Get the truck scanned and post the code(s) and we can help you from there.

blazes9395
01-29-2010, 12:14 AM
DMB359, I see you do have codes....check for vaccuum leaks as I mentioned, test the IAC like I mention too to verify the IAC is operational. If everything looks good after the above is checked, get a fuel pressure guage on there and check runnin pressure and leak down .

Holy smokes......I didn't check the dates of the original post.......almost a year!, Well I guess all this is here for reference, Bartone hopefully it will help you......

Bartone_ml
01-29-2010, 04:48 PM
Ok -

As you figured out DTC 35 is the only code.

I did check for vacuum leaks today and couldn't find any.

I suspect I have a fuel pressure issue and need to change the fuel filter. I suspect this is the issue as I have two other vehicles that had a clogged fuel filter. (Grrrr... Picking up bad gas somewhere...) However, I don't understand how that will cause a fast idle issue.

Thanks in advance. Mike

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