Higher than normal Idle, and occasional stumble

[Billy Mild]

I have been struggling with a higher than normal idle since I pulled the plenum a few months ago. When I pulled the plenum the following things were replaced: Plugs, Wires, Fuel Inejctors, and fixed any remaining vacuum leaks.

Sometimes my idle will hang around 1500-1700 RPM. If I stab the throttle really fast it will go back down to 900 RPM. I have noticed that when the engine is still cold, not fully warmed up, the car will sometimes stumble around the 2500 RPM when I go more than 50% throttle. If I go WOT it pulls through it without issue. There seems to be a sticking point around 50% throttle. Once the car comes up to full temp this issue becomes less noticeable. I haven't tried to spray the throttle blades with lithium grease or anything.

I have read a few old threads concerning the idle issues. It seems that may be related to my Throttle Body needing a service, or there could be a vacuum leak somewhere. I don't think there is a vacuum leak since my secondary vacuum pump will run for a few seconds with the key on, and shuts off for a long time. I tried to do the IAC reset procedure. I drove just a little bit after that and the idle did seem better, but still not perfect.

Any ideas?

[Paul Workman]

I'm betting it is TB/linkage cable related, in that if you poke the throttle the idle will sometimes come down to normal - which it would not if it were a plenum gasket leak or the like.

First, make sure the connections in the TB connector are clean and the connector itself is attached properly. Then, with the switch on, but the engine not running, the TB voltage signal at ilde position should adjusted to .54v, ± .08V (.46 to .62 volts)*

*1990 FSM; Driveability and Emissions, sec. 6E3-C1-19 (TPS) Adjust Note: The voltages may might vary for other years, I don't know.

Next we want to verify the IAC valve connection is properly seated. And, if it is, then pull the IAC valve and inspect the pintle and the air horn seat to be sure they are not impeded with carbon deposits, or the like and the pintle moves freely before reinstalling it.
If all checks out to this point, then the linkage and connections, and the throttle plates too should be checked out to verify proper motion - NOT sticking, and that includes the idle throttle plate as well.

Note: There is a waxy seal spread around the throttle plates to help seal them when closed. It must NOT be removed, or idle issues will prevail. Be careful to not disturb it.

While idling, place a book or the like across the open air horn. If it continues to idle, then for sure you have a leak - likely a plenum gasket got dislodged or the like. But, that is more for reference at this time, until you've eliminated the TB/IAC functions first.

BTW, the vacuum pump not running/shutting off does NOT mean there is no leak elsewhere, e.g., say the plenum gasket, for one. The secondary actuator circuit is isolated from the plenum by check valves. The pump will "charge" the vacuum circuit initially and then shut off, even with ZERO vacuum in the plenum, e.g., such as when the key is turned on but the engine is not running, OR at sustained WOT operation.

[XfireZ51]

Paul,

The TPS v should be .53-57v with key ON/no engine start.

[Paul Workman]

Paul,

The TPS v should be .53-57v with key ON/no engine start.

Not to split the atom, but I wonder if the voltage recommendation varies w/ model year? But, in any case, mid .5s volts appears to be close and easy to remember, I recon.

[XfireZ51]

:USFlag:Not to split the atom, but I wonder if the voltage recommendation varies w/ model year? But, in any case, mid .5s volts appears to be close and easy to remember, I recon.


Not sure why it would vary by model year. Same TPS, same values for idle in the calibrations. There is provision for hysteresis in the cal also.

[Billy Mild]

I don't have a scanner. Is there another way to check the the tps voltage and then IAC counts?

[mike100]

I don't have a scanner. Is there another way to check the the tps voltage and then IAC counts?

IAC counts can't be determined without a scanner, but the TPS voltage can be manually read by carfully probing the center wire of the tps connector with a sharp instrument or paperclip possibly and then to ground on the other side of the meter (or to the negative wire on the edge of the 3 wire connector). Back-probe the connector with the ign off and then once the meter is all hooked up, then turn it on.

[Billy Mild]

Well it looks like I am back to square one. I drove the ZR1 quite a bit yesterday and my secondary pump would only prime the system then never run again. This morning (34 degrees) it would prime, then stop running for 5 seconds, then start up again. I guess I have a vacuum leak again under the plenum.

Is there anyway to fix these vacuum leaks for good? I'm considering using silicone tubing like I used on my old 944 Turbo. After I used that stuff I never had vacuum leaks again.

[Dynomite]

Well it looks like I am back to square one. I drove the ZR1 quite a bit yesterday and my secondary pump would only prime the system then never run again. This morning (34 degrees) it would prime, then stop running for 5 seconds, then start up again. I guess I have a vacuum leak again under the plenum.

Is there anyway to fix these vacuum leaks for good? I'm considering using silicone tubing like I used on my old 944 Turbo. After I used that stuff I never had vacuum leaks again.

I am assuming you are removing the Plenum........What usually happens is that the little rubber connectors get hard or loose which you can use your silicon Seal. Some use a nylon strap tightened around the fitting. Others like myself.....find a nice rubber connector that fits tight. Jerry has some of those connectors in stock. Vacuum System Components (http://www.jerrysgaskets.com/search.php?search_query=vacuum+system&x=0&y=0)

Finding A Vacuum Leak (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-7.html#post1584987830)

Now if you really want to have some fun and be able to remove the Plenum in less than 10 minutes and fix those secondary vacuum leaks in just a few minutes....... ELIMINATE TB COOLANT ;)

See Item #4 LT5 Eliminated Systems (http://forums.corvetteforum.com/c4-zr-1-discussion/2942569-tech-info-lt5-eliminated-systems.html)

[Billy Mild]

I am assuming you are removing the Plenum........

Finding A Vacuum Leak (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-7.html#post1584987830)

This will be my 6 or 7th time taking the plenum off. Granted 3 of those times were within the same day since I forgot something or left a connector off.

I should be able to get this done pretty quickly. I'm assuming somehow one of the hoses vibrated off somehow. I may end up replacing the rubber ends with silicone hose. Follow this procedure to make sure they can't come off.
http://www.turbo944.com/bbs/config.cgi?noframes;read=8056

Might be crazy, but it works. I am tired of the vacuum leaks.

[Billy Mild]

I just drove the Vette again, the engine was a bit warmed up and it appears the secondary pump is behaving like it was yesterday. Run with the initial key being on, then shut off for a while.

Who knows maybe its temp related.

[Dynomite]

I just drove the Vette again, the engine was a bit warmed up and it appears the secondary pump is behaving like it was yesterday. Run with the initial key being on, then shut off for a while.

Who knows maybe its temp related.

Does this mean your idle is 750 to 800 and smooth?
Does this mean your Vacuum Pump turned on for a few seconds with switch on and then turned off for at least a minute or so?
Keeping in mind if the Vacuum Pump turns on for a few seconds and then Off for a few seconds you still have operational Vacuum for Secondaries.

Operational Vacuum with Leaks but not something that will effect secondary operation. Something rather normal but not perfect.
Simple to get the Vacuum System Perfect ......:D

Finding A Vacuum Leak (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-7.html#post1584987830)

The higher than normal idle and stumble is NOT normal. Yes higher than Normal Idle for a few seconds (or until vehicle forward motion is zero) as that is programmed for idle to be a bit dependent on vehicle motion forward (speed). We are talking Out Of Gear (or clutch depressed) and foot off gas coasting at engine idle ;)

This has already been suggested that your MAP connections at rear of Plenum are connected and tight.

[Billy Mild]

Does this mean your idle is 750 to 800 and smooth?
Does this mean your Vacuum Pump turned on for a few seconds with switch on and then turned off for at least a minute or so?
Keeping in mind if the Vacuum Pump turns on for a few seconds and then Off for a few seconds you still have operational Vacuum for Secondaries.

Operational Vacuum with Leaks but not something that will effect secondary operation. Something rather normal but not perfect.
Simple to get the Vacuum System Perfect ......:D

Finding A Vacuum Leak (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-7.html#post1584987830)

The higher than normal idle and stumble is NOT normal. Yes higher than Normal Idle for a few seconds (or until vehicle forward motion is zero) as that is programmed for idle to be a bit dependent on vehicle motion forward (speed). We are talking Out Of Gear (or clutch depressed) and foot off gas coasting at engine idle ;)

This has already been suggested that your MAP connections at rear of Plenum are connected and tight.

My idle will be 750-800 RPM sometimes. Other times it will be around 900-1000 RPM. I have noticed that if I stab the throttle and that brings the RPM's up to around 2200 RPM and just let it drop. The idle will go down to 750 RPM Range. Something tells me that has to do with the TB itself possibly.

The Vacuum Pump early this morning when it was really cold, would run then stop for a few seconds, then run, run a second or so then run. After I drove the car around and it warmed up a bit, it seemed to not run as often more what I am used to after the vacuum leaks were fixed this past summer.

I know the car should have the idle high when coasting down to a stop with the car out of gear. My issue is when I am at a dead stop.

Stumble is hard to explain without actually driving the car and feeling the throttle throughout the range. I need to do some slow driving around the neighborhood and see if the throttle plates are sticking.

[Billy Mild]

It seems my car has a vacuum leak all the time now. My vacuum pump does cycle, but it does it when warmed up as well.

Can you still get replacement rubber ends for the spider of vacuum hoses? Should I try replacing them all with silicone tubing? I want to pull the plenum and it be the last time. So I am going to block off the water if possible, replace the ignition coils, and hopefully not have any vacuum leaks after this time.

[WARP TEN]

This will be my 6 or 7th time taking the plenum off. Granted 3 of those times were within the same day since I forgot something or left a connector off......

Ha Ha sounds like the way I approach projects. You get to practice a few times before getting it done right....Bob

[Dynomite]

It seems my car has a vacuum leak all the time now. My vacuum pump does cycle, but it does it when warmed up as well.

Can you still get replacement rubber ends for the spider of vacuum hoses? Should I try replacing them all with silicone tubing? I want to pull the plenum and it be the last time. So I am going to block off the water if possible, replace the ignition coils, and hopefully not have any vacuum leaks after this time.

How often does your pump cycle? (How long is it running and how long is it off during the cycles)? Finding A Vacuum Leak (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-7.html#post1584987830)

You should be able (with plenum off) to find and close ALL vacuum leaks so the pump cycles only once within a minute or so. Do NOT replace the Plenum until you get to that point.

See Injector Housing TB Coolant Blocking (http://www.corvetteforum.com/forums/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-10.html#post1588695506) This post was upgraded today as I just blocked TB Coolant on two Injector Housings. This is the non-transparent method as no one will be able to tell TB Coolant is blocked.

Injector Housing TB Coolant Blocking

Remove Plenum and Block TB Coolant at Injector Housing (Undetected and appears stock or otherwise knows as Non-Transparent).
Install 1/8 inch NPT allen head pipe plugs (21/64 or 11/32 drill) two each Injector Housing (IH) Coolant as shown.

Install the two 1/8 inch x 27 NPT Aluminum pipe plugs in the Injector Housings. See Summit Racing 1/8 inch Aluminum Pipe Plugs (http://www.summitracing.com/parts/SUM-G1485R/) for Aluminum Pipe Plugs.

It is simple and all you have to do is place 1/8 inch pipe plugs in two ports after drilling out a bit. You do NOT need a drill press but try to drill and tap as perpendicular to the Injector Housing Intake surface as possible. Keep trying the pipe plug as you tap the hole such that the pipe plug becomes tight but almost flush with the Injector Housing/Plenum mating surface. Use Red Loctite on the pipe plug in the final installation (the final installation procedure is to take a flat file and dress up the pipe plug with the surface (the pipe plug you use is Aluminum so easy to dress up).

It is important to have a shop vac handy as you drill and tap into aluminum to suck up all shavings.

Note 1: When drilling into aluminum with hand drill do NOT apply a lot of pressure and trigger the drill on and off rather than just hold the trigger. In aluminum the drill bit might catch and then the hole you are drilling gets screwed up just as you are about to pop through. In this case you are about to pop through at the start as the smaller hole is already there. If you are not sure.....chuck up the drill bit lightly so it spins in the chuck if it takes a bigger bite than expected when drilling.

Note 2: When tapping the drilled hole make sure the tap is perpendicular to the Injector Housing surface. Turn the tap with vertical pressure to start the thread and then rotate the tap only one turn at a time backing off and cleaning the tap each complete turn. As you get close to the tap depth required, keep trying the pipe plug such that it is near flush when finally installed but with considerable resistance when installing. The final installation is with red Loctite and then file smooth with the surface.

http://i287.photobucket.com/albums/ll142/dynomite007/Dynomite10/96f18e01-b82a-480b-882d-4bda78747ffb.jpghttp://i287.photobucket.com/albums/ll142/dynomite007/Dynomite10/b0a364ba-d5c6-439c-9e30-f7ea1da2d528.jpg
http://i287.photobucket.com/albums/ll142/dynomite007/Dynomite10/7ebd6092-fef7-4069-8529-3946c348685d.jpghttp://i287.photobucket.com/albums/ll142/dynomite007/Dynomite10/2ba39678-4446-4eb4-a1c8-21762a216b56.jpg

This Non-Transparent Installation of TB Coolant Blocking
will require you insert a plug in the Coolant Return
Line on the Passenger side of the Plenum
(the only TB Coolant hose that requires a hose clamp).

http://i287.photobucket.com/albums/ll142/dynomite007/Dynomite9/9eabe7b5-5f0c-48eb-aa6d-835c16de2400.jpg

Note 3: Getting the Air Out of the Water Pump after Blocking TB Coolant.
See LT5 Air Locked Water Pump Simplified (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-6.html#post1584756847) for the Simplified Discussion regarding The Air Locked Water Pump.

1. Disconnect the drivers side Injector Housing Coolant Manifold "L" and tip it up. Fill that "L" with coolant and blow into that "L" forcing coolant into the water pump through the passenger side water pump inlet and passenger side Injector Housing Coolant Manifold (Only blow in steps refilling the "L" with coolant each blow effort). You can see in the photo that by blowing coolant into the Drivers Side of the water pump air will be forced out to the top level of the impeller with only a small amount of air left on the passenger side of the water pump.

2. Refill the Drivers Side "L" with coolant and do that trick a couple times until the coolant flows out of the drivers side Injector Housing Coolant Manifold where you have disconnect the "L".

http://i287.photobucket.com/albums/ll142/dynomite007/C%20Engine%20LT5/947bedc9-e7b2-4998-9234-45830bc8f7e7.jpg

[Billy Mild]

How often does your pump cycle? (How long is it running and how long is it off during the cycles)? Finding A Vacuum Leak (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-7.html#post1584987830)

You should be able (with plenum off) to find and close ALL vacuum leaks so the pump cycles only once within a minute or so. Do NOT replace the Plenum until you get to that point.



My car was like that at one point. It was after I removed the plenum for the first time, and the subsequent times after that. The vacuum leak issue seems to come and go. For example after a lot of driving today when the engine was still warmed up, the pump didn't cycle at all. Pretty crazy.

The plenum is coming off soon for new coil packs, as it appears my car may have a slight misfire just in the idle to 2500 RPM range.

[Dynomite]

The vacuum leak issue seems to come and go. For example after a lot of driving today when the engine was still warmed up, the pump didn't cycle at all. Pretty crazy.


I regard to a vacuum leak that comes and goes.........as indicated by a pump that cycles some times much more often that other times.

A possibility not yet discussed.........

There is a Vacuum Check Valve under the Plenum that isolates the Vacuum Pump from the Plenum Vacuum as long as the Vacuum Pump is pulling greater vacuum than the Plenum.

With the Key On but engine NOT running the Vacuum Check Valve can be tested for proper operation without removing the Plenum as follows........

The Plenum source of vacuum is located on the drivers side center of Plenum. Disconnect that Vacuum Connector from the Plenum and place finger over end of vacuum line rubber connector to see if the Vacuum Pump quits cycling. With Key ON the Vacuum Pump will cycle every two or three seconds with that vacuum rubber connector NOT blocked and failed vacuum check valve. With that Vacuum Rubber Connector blocked if the Pump now stops cycling you have located the Vacuum Leak which actually is a faulty check valve located under the Plenum. The Plenum has to be removed to replace that check valve.

This faulty check valve can fail intermittently and will then cause intermittent operation of the Vacuum Pump. The test as described is a simple test and can be conducted with the Plenum in place.

Finding A Vacuum Leak (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-7.html#post1584987830)

[secondchance]

Sometimes my idle will hang around 1500-1700 RPM. If I stab the throttle really fast it will go back down to 900 RPM. I have noticed that when the engine is still cold, not fully warmed up, the car will sometimes stumble around the 2500 RPM when I go more than 50% throttle. If I go WOT it pulls through it without issue. There seems to be a sticking point around 50% throttle. Once the car comes up to full temp this issue becomes less noticeable.

Have you checked your throttle position sensor? My symptom was quite similar. Turned out TPS going bad.

[Billy Mild]

I haven't yet. It sounds like I need to get a hold of a tech1 and check that stuff.

Dynomite I have replaced that check valve BTW. It seems that when its warm outside and my engine is warmed up the vacuum pump doesn't run all the time. So it is intermittent.

Would it be a bad idea to replace all vacuum lines with silicone hoses?

[XfireZ51]

I haven't yet. It sounds like I need to get a hold of a tech1 and check that stuff.

Dynomite I have replaced that check valve BTW. It seems that when its warm outside and my engine is warmed up the vacuum pump doesn't run all the time. So it is intermittent.

Would it be a bad idea to replace all vacuum lines with silicone hoses?

The hard plastic lines are there to avoid collapse during high vacuum operation.

[Dynomite]

The hard plastic lines are there to avoid collapse during high vacuum operation.

Concur :thumbsup:

I have found in addition to that check valve failing, that the usual vacuum leaks are caused by the rubber connectors getting hard and shrink with age on the ends of the hard plastic lines. If you cannot remove the rubber connectors they probably do not need to be removed. It is the loose rubber connectors that have shrunk and leak. Or.....have disconnected themselves from the hard plastic lines.

Be careful that you do not kink the hard plastic lines by excessive bending.

It seems that when its warm outside and my engine is warmed up the vacuum pump doesn't run all the time. So it is intermittent.


That fits right in with the connectors getting a bit tighter as they warm up.

[secondchance]

I haven't yet. It sounds like I need to get a hold of a tech1 and check that stuff.

Dynomite I have replaced that check valve BTW. It seems that when its warm outside and my engine is warmed up the vacuum pump doesn't run all the time. So it is intermittent.

Would it be a bad idea to replace all vacuum lines with silicone hoses?

You don't need Tech 1 to check. I believethere are three wires coming out of TPS. Use paper clip inserted on top and bottom where wires are connected (I belive the middle is reference voltage). Turn the key to run position (don't start the motor) and read voltage between the top and bottom wire. At idele it should be around .55 volts. Have someone dpress the gas pedal and see if the voltage increases gradually. If you see voltage fluctuating, you got a bad TPS.
Typically (perhaps not always), vacuum leak from the secondary vacuum system does not cause high idle nor stumbling based on my experience.

[Billy Mild]

Can you get those rubber pieces new?

[secondchance]

Can you get those rubber pieces new?

You can't get OEM rubber pieces only. However, Advance Autoparts has an aisle with assortment of plastic vacuum lines, fittings and rubber hose ends.

[RussMcB]

Be careful that you do not kink the hard plastic lines by excessive bending.Concur. I surprised myself when one broke in two easily when I was trying to move it off to the side while working. Luckily I was removing secondaries, so it wasn't a big deal. I guess they can get pretty brittle in that environment after a few decades.

[XfireZ51]

Concur :thumbsup:

I have found in addition to that check valve failing, that the usual vacuum leaks are caused by the rubber connectors getting hard and shrink with age on the ends of the hard plastic lines. If you cannot remove the rubber connectors they probably do not need to be removed. It is the loose rubber connectors that have shrunk and leak. Or.....have disconnected themselves from the hard plastic lines.

Be careful that you do not kink the hard plastic lines by excessive bending.



That fits right in with the connectors getting a bit tighter as they warm up.
I spread a bit of silicone gasket sealer around the joint of the hard line and the rubber connector.
Will also use tie wraps around that area.