"mimimum idle speed"

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T-Max

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1994 Chevy Suburban 5.7 liter K1500 TBI

Does anyone know what the "minimum idle speed" is for this truck? (EDIT: Actually, what I guess I'm looking for is the “target idle speed” which is set by the chip).

I am in New Jersey and having trouble passing the emissions test (@ idle) because the hydrocarbon reading is too high. It passes the "high idle" test (which I guess is just basically with the engine racing) but won't pass the test at idle.

It is idling at 400 rpm in neutral (park) when fully warmed up. That seems appropriate but I believe that if I set it higher, I may pass the test.

The following (very informative) link gives the procedure for setting the idle speed and refers to the “target idle speed” which supposedly is given in the service manual.

http://www.gmcmidwestclassics.org/Web pages/Tuning the TBI.pdf

I did a search on the site for this term but got no hits

Thanks for any help you can give.

:)
 
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zora said:
To me that speed, 400 rpm, seems extremely low. The thread below points out that the IAC may need to be cleaned, did you do that? This article has a number of references to 625 and above as the correct speed.

http://www.chevytalk.org/fusionbb/showtopic.php?tid/286541/
Click to expand...

Thanks much for the reply and especially for the thread link. I agree that the idle seems quite low. That site that I linked shows the table (on page 4) indicating that @ 55 degrees C, the "target idle" should be 750 rpm. On the thread you linked, Vaughn says that "The idle specification for a 5.7L tbi is 600 rpm in neutral on a manual trans, 550 for an automatic in drive."

My truck at idle in drive barely registers on the vehicle tach. I don't have any idle or stalling issues, and have no problem with it the way it is, but I gotta get it thru inspection and I think it needs to be speeded up at idle.

I have not as yet cleaned the IAC, but I am going to do that.

Also, if you check out my prior posts earlier this year, you'll see that I had a VERY rich running condition which was due to a bad CTS. I drove the truck approximately 50 miles with that very sooty condition before identifying and correcting the problem, and just that amount of driving coated the end of my relatively new tailpipe with pretty thick black soot.

So there's that to consider. I would imagine my O2 sensor got pretty sooty (tho' I would hope it's burned clean by now).

It's been suggested that a "lazy" O2 sensor could cause a high HC condidtion. But I'm okay at "high idle" (about half of the standard), so I wouldn't think the O2 sensor would be the problem.

I'll clean the IAC and re-set the idle per the procedure in the document I linked. Hopefully, that'll cure the problem.

Thanks again.
 
I guess you have no engine codes?
 
zora said:
I guess you have no engine codes?
Click to expand...

Correct. No error codes showing up. I checked just before going into the inspection station and also immediately after coming out (in case the test set a code). No codes either time.

The truck runs very well; it's just giving a high HC reading at idle. Cleaning the IAC and re-setting (and increasing) the idle might be all that's required.

In any event the re-test will give me the results. The "standard" for HC in NJ is 220 ppm. I scored 597 the first time, and 680 the second time (after putting some high test gas in at the inspector's suggestion).

The "high idle" standard is the same (220 ppm) and on that test I scored 131 the first time and 116 the second time. So those levels are quite good.
 
zora said:
To me that speed, 400 rpm, seems extremely low. The thread below points out that the IAC may need to be cleaned, did you do that? This article has a number of references to 625 and above as the correct speed.

http://www.chevytalk.org/fusionbb/showtopic.php?tid/286541/
Click to expand...

I went to my local library and checked out the Chilton manual. It has a blurb on the Idle speed adjustment and says to "Adjust the idle stop screw to obtain the specified RPM reading"

When you look at the tune up specs table, that says (referring to the correct idle speed): "refer to underhood label for exact setting"

My underhood label does not give any RPM value for the idle speed; it simply says that the idle speed is automatically controlled.

BUT, Chiltons also has a "Minimum Idle Speed" table (@ page 4-52 of the relevant manual) which says that for an automatic transmission vehicle, the idle speed for a 5.7 L engine (under 8500 GVW) in Drive (not Neutral), with the ECM in "closed loop", should be 425 RPM +/- 24 RPM.

I'm not sure how to determine that the ECM is in "closed loop", so if anyone could 'splain that to me, I'd appreciate it.

In any event, my 'burban in Drive (fully warmed up) is not idling at anywhere near 425. It's basically at that RPM in neutral, but not in Drive

So that's what I know at this point.

I'm currently removing the IAC to clean it
 
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This explains closed/open loop, basically it means on a properly functioning engine when at full operating temperature that is the RPM. So in neutral it would go up at least 200 rpm which would put it in the range mentioned in the other thread. Curious if it will self correct after the IAC is cleaned.

http://www.thirdgen.org/open-loop-closed-loop
 
zora said:
This explains closed/open loop, basically it means on a properly functioning engine when at full operating temperature that is the RPM. So in neutral it would go up at least 200 rpm which would put it in the range mentioned in the other thread. Curious if it will self correct after the IAC is cleaned.

http://www.thirdgen.org/open-loop-closed-loop
Click to expand...

Thanks much for that link. Yea, sounds like "closed loop" just means the engine is fully warmed up, which is the only proper time to adjust the idle speed since all sources say to fully warm the engine before performing the procedure on the IAC to "adjust the idle speed" / "calibrate the TBI".

I cleaned the IAC today and that was straightforward. The only "problem" was that the gasket completely fell apart -- it broke into about 6 or 7 pieces. I made a gasket out of some cardboard of the approximate same thickness which I may or may not replace later with some proper gasket material. In any event, on reinstalling the IAC, I snugged it up to basically the exact position it was in before I removed it.

Which I think is probably important for correct pintle positioning/extension.

I'll let you know how things go after I perform the IAC procedure.

P.S. - The IAC had some significant soot on it, but I dunno if that would make any difference at all in its performance. I suspect not, but it certainly doesn't hurt to clean it.
 
Just to update things, I changed the engine oil and filter to eliminate the possibility that dirty oil was contributing to the high HC reading at idle.

I then went through the procedure to re-set the idle speed and ended up making NO adjustment at all because things seem to be in line with specs. But I'm not sure about that, as follows.

I warmed the truck up for 20 minutes to nearly 160°. Idle speed in Park was 400 RPM. I did the steps as per both Chiltons and the online TBI document I linked in my opening post, and on restart with the IAC disconnected idle speed was 700 RPM in Park. In Drive it was 400 RPM. Since the "Minimum Idle Speed" from the Chiltons table is 425 in Drive (+-25 RPM), that seems to be right on target because I believe that table is for when you have a "seated IAC valve." But I'm not sure about whether or not that idle speed is for when you have a seated IAC valve or for under normal operating conditions. Under normal operating conditions my idle speed in Drive is 200 RPM, which would be at least 200 RPM too low.

Note that the online TBI document says that at 160° (~70° Centigrade), the “target idle speed” (set by the chip) is 700 RPM. So if that means the “target idle speed” is the idle speed in Park with a seated IAC valve, I'm right on. BUT, the online document also says that "The RPM that the engine is idling at now is probably what the RPM is set for in the current chip that you have." So that suggests to me that I should be idling at 700 RPM in Park w/ a warm engine under normal operating conditions. If so, I'm 300 RPM below that and should be kicking the RPMs up. BUT, that document also says (essentially) that if you set the idle too high during the procedure, damage the IAC motor could occur, and I obviously don't want that to happen. Plus, that idle speed (700 RPM in Park w/ a warm engine) seems a tad fast to me.

Anyway, that's what happened with the procedure to set the idle speed.

Now, in performing this procedure, I noted that the engine had a "skip" at idle, which was very noticeable in Drive (vehicle lurched a bit). I ran the engine after dark to observe any sparking in the plug wires, and saw nothing.

So I removed the distributor cap and rotor (which are relatively new) and cleaned them up a bit, then I also wiped the inside of the distributor cap clean as well. After doing that, the "skip" went away and I went to inspection. It failed again with a reading of 503 at idle (needs to be 220 or below). It passed again at "high idle."

So the first failed reading was 597; then the 2nd reading (after adding some premium gas) was 680, and this last reading (after changing the oil and filter and cleaning up the distrib cap and rotor) was 503.

So that's where things currently stand.

Now, I will say that after pulling out of the inspection station, I immediately pulled over and checked that butterfly valve in the air cleaner which is normally closed when the engine is cold so that warm air from the exhaust manifold can be drawn to warm the air drawn into the TBI. That valve was completely closed even though the engine was fully warmed up. That obviously shouldn't be, so I will make sure that valve is open the next time I go to inspection. Whether or not that will make any difference is to be determined.

I think the guys at the inspection station are getting tired of seeing me, but I'll continue to fiddle until I run out of options, at which point I'll have to take it to a private place to see what they have to say about it.

I'll check out the EGR valve next, correct a possible PCV vacuum leak (which I don't think is leaking), perhaps retard the timing a bit, and then I think I'll be out of tricks to try to get this beast through inspection.

If anybody has any thoughts on my current idle speed, and what your idle speed is in both Park and Drive, and what you think about that, feel free to share.
 
You should have a 195 degree thermostat, especially if you are trying to pass emission testing, is that what you have?
 
zora said:
You should have a 195 degree thermostat, especially if you are trying to pass emission testing, is that what you have?
Click to expand...

Don't know what thermostat is in there. I certainly could get a 195 thermostat and put it in, so maybe I'll do that.

My temp gauge is spotty; sometimes it works and sometimes it doesn't. As I said, I ran it for 20 minutes and the gauge got up to 160 but no higher. Whether or not that was actually 160, I can't say.

The truck puts out good heat in the winter, which is all I know about the thermostat situation.

It seems a bit odd to me that the truck easily passes "high idle" but fails @ idle. The high idle standard is also 220 ppm (same as the "idle" standard) and the first reading was 131, the 2nd reading was 116, and the last reading was 165. So the high idle HC readings are all well under the standard. I'm thinking that maybe the last reading was high because that butterfly valve that should have been fully open was pretty much fully shut.
 
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