High RPM's on start up

[joshwilson3]

On my 89, it was down for a month or so. I had to replace a rear hub, ujoints, power steering pressure hose, air pump check valve, and spark plug wires.

After that, I went to start it. But turned the key and nothing happened. Must have been the VATS thing I guess as I came back out an hour later and it started up.

But since I had started it back up, I've noticed a high RPM spike on a cold and warm start up. On start up, the RPM's shoot up, then take several seconds to fall back down. Before I realized what was going on, on a warm start up I put it in gear fast before the RPM's dropped down. And it kicked into gear hard.

I don't notice anything odd after it is in gear. Just on a start up, but more noticable on a warm start up. I see the RPM's shoot up to 2200 or so, but also have seen 4400. But that could be an error on the digital dash.

A couple of times on a warm start up, the RPM's hunted up and down till I put it in gear.

Last year, I pulled the throttle body to clean it. The IAC looked new. I reset the TPS and checked it to make sure it didn't have any dead spots. Replaced the PVC valve and 3-way check valve. Fuel filter has been replaced as well as spark plugs.

 

[TedC]

Those are pretty high spikes. I see 1100 RPM when starting cold below 50*F and 900 RPM between 60-80*F when starting cold. Would check the TPS, making sure voltages are correct at idle, WOT, and smooth in between. Would also reset the IAC and ECM. The ECM processor used in L98s is not that fast (cycle times), and a responsive IAC is key to smooth idle performance. Have you checked for vacuum leaks? You may find this helpful...

Throttle Body Adjustments for 1989 L98 Corvette

Start and run engine until it reaches operating temperature (closed loop)
Check and set engine timing to 6 degrees of advance with EST wire disconnected
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Jumper terminals "A" and "B" on the ALDL
Turn ignition key on and do not start engine
Wait 60 seconds so that the idle air control (IAC) motor fully extends
Without turning ignition key off, remove connector from IAC motor
Turn ignition key off and disconnect ALDL jumper
Start and run engine until it reaches operating temperature (closed loop)
Remove minimum idle air cap if required
Adjust idle speed to 425 (+/- 25) rpm in either in park or neutral
Turn ignition key off and reconnect EST wire and IAC motor
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Reset ECM by disconnecting and reconnecting power at battery terminal
Depress accelerator pedal slightly
Start and run engine for 5 seconds
Turn ignition key off for 10 seconds
Drive vehicle to assist in ECM reprogramming

 

[joshwilson3]

Those are pretty high spikes. I see 1100 RPM when starting cold below 50*F and 900 RPM between 60-80*F when starting cold. Would check the TPS, making sure voltages are correct at idle, WOT, and smooth in between. Would also reset the IAC and ECM. The ECM processor used in L98s is not that fast (cycle times), and a responsive IAC is key to smooth idle performance. Have you checked for vacuum leaks? You may find this helpful...

Throttle Body Adjustments for 1989 L98 Corvette

Start and run engine until it reaches operating temperature (closed loop)
Check and set engine timing to 6 degrees of advance with EST wire disconnected
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Jumper terminals "A" and "B" on the ALDL
Turn ignition key on and do not start engine
Wait 60 seconds so that the idle air control (IAC) motor fully extends
Without turning ignition key off, remove connector from IAC motor
Turn ignition key off and disconnect ALDL jumper
Start and run engine until it reaches operating temperature (closed loop)
Remove minimum idle air cap if required
Adjust idle speed to 425 (+/- 25) rpm in either in park or neutral
Turn ignition key off and reconnect EST wire and IAC motor
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Reset ECM by disconnecting and reconnecting power at battery terminal
Depress accelerator pedal slightly
Start and run engine for 5 seconds
Turn ignition key off for 10 seconds
Drive vehicle to assist in ECM reprogramming

I set the TPS last year, and checked it again. It is fine.

I've never messed with the idle air since it always idled fine.

It only does that RPM spike on start up only. When in gear, driving and idling it is fine.

It never did the RPM spike until after it sat for a month when I did some things.

I say I could look for a vacuum leak, but don't see how one could have came all of a sudden. But I figured if there was a vacuum leak that it would affect the idle when in gear and not just on a start up?

 

[Nick90vetteguy]

I believe most Hi-Rpm rev's from start up are poor MAS sensor, possibly a bad cable setup/linkage from your throttle body to tranny( A good tip to reset it is start your vette let it idle, shut it off. take the key out of the ignition, step on the gas Dealerships charged me 80.00 bucks to reset it before I found out :W Another issue maybe be your gas pedal(bar) is stuck.

 

[joshwilson3]

I believe most Hi-Rpm rev's from start up are poor MAS sensor, possibly a bad cable setup/linkage from your throttle body to tranny( A good tip to reset it is start your vette let it idle, shut it off. take the key out of the ignition, step on the gas Dealerships charged me 80.00 bucks to reset it before I found out :W Another issue maybe be your gas pedal(bar) is stuck.

When you say "MAS sensor" do you mean MAF?

I'll try that reset thing you mentioned. And I guess I could also try holding the gas pedal back when I start it to see if that does anything. But every time I've been under the hood, the linkage at the throttle body is touching that screw, so my guess is it is all the way closed and not being left partly opened.

 

[TedC]

If you suspect a MAF issue, then you can disconnect it and see if the idle quality changes. When checking/setting TPS voltages, the best way is using a scanner and using the gas pedal to move the throttle cable from idle to WOT. This is the best procedure because you monitor the same voltages the ECM is seeing. Others here on CAC have posted idle issues caused by poor TPS connections, yet the TPS voltages looked AOK. Without a scanner, use a DVOM connected to TPS but viewable from driver's seat. Move the TPS using the gas pedal not hand twisting of throttle plates. Make sure WOT exceeds 4 volts (I like to see 4.5V+). If the throttle cable is sticking, you will quickly discover it. I provided the procedure to reset the ECM and IAC.
Reset ECM by disconnecting and reconnecting power at battery terminal
Depress accelerator pedal slightly
Start and run engine for 5 seconds
Turn ignition key off for 10 seconds

 

[joshwilson3]

If you suspect a MAF issue, then you can disconnect it and see if the idle quality changes. When checking/setting TPS voltages, the best way is using a scanner and using the gas pedal to move the throttle cable from idle to WOT. This is the best procedure because you monitor the same voltages the ECM is seeing. Others here on CAC have posted idle issues caused by poor TPS connections, yet the TPS voltages looked AOK. Without a scanner, using a DVOM connected to TPS but viewable from driver's seat. Move the TPS using the gas pedal not hand twisting of throttle plates. Make sure WOT exceeds 4 volts (I like to see 4.5V+). If there the throttle cable is sticking, you will quickly discover it. I provided the procedure to reset the ECM and IAC.
Reset ECM by disconnecting and reconnecting power at battery terminal
Depress accelerator pedal slightly
Start and run engine for 5 seconds
Turn ignition key off for 10 seconds

Thanks, I'll check the TPS out again with the gas pedal. When I checked last, I used my hand to turn the blades.

I'll do those other resets you mentioned as well. I do know I had the battery disconnected for a month while I was working on other things before I noticed the high rev on start up.

I'm sure I'll probably figure it out, but it may take some time since it is just a high rev at start up, and then the RPM's drop down like they are supposed to. And I don't notice any problems with idle when in gear.

 

[Nick90vetteguy]

MASS air flow, MAF sensor same difference :booty:booty hmmm..its touching a screw, doesn't sound right, it shouldn't be "sagging", nor should it be touching anything, it should be nice and taught. Did you take off/remove the cables before? I never had a High rpm rev, mine sits at a nice 1500 idle, try starting your car shift all gears one-by-one. I know when I installed a BBK 58mm throttle Body I had a slight high rpm rev, but I let the vette run for about 4min and it managed to find its groove back into 1500, but for the most part soounds like a cable problem, which is very lucky for you :eyerole by the way is your crusie control on?

 

[Nick90vetteguy]

When you say "MAS sensor" do you mean MAF?

so my guess is it is all the way closed and not being left partly opened.

If thats so the case, there should be what they call a IDLE SCREW on the top right corner of your throttle body, it should be screwed in a hole, theres some advantages/disadvantages to this 1 being if you have a stock TB(Throttle body) over the course of years(wear-n-tear) will set in place from heat and coolness ie: expansion and contracting and it shouldnt move if the screw is more than a 1/4 inch or 3/8 out facing toward the front of the car that COULD be a problem just screw it back in 1/4 of a inch or to be safe 3/8, I know my current TB sometimes it moves from the vibration and bumps, etc. I JB'd that b*tch back on and hasnt moved since if you'd like i can take a pic of my STOCK TB and send ya pic how it should be positioned, anything over 1/4 on the screw is for different types of tunnings

 

[joshwilson3]

MASS air flow, MAF sensor same difference :booty:booty hmmm..its touching a screw, doesn't sound right, it shouldn't be "sagging", nor should it be touching anything, it should be nice and taught. Did you take off/remove the cables before? I never had a High rpm rev, mine sits at a nice 1500 idle, try starting your car shift all gears one-by-one. I know when I installed a BBK 58mm throttle Body I had a slight high rpm rev, but I let the vette run for about 4min and it managed to find its groove back into 1500, but for the most part soounds like a cable problem, which is very lucky for you :eyerole by the way is your crusie control on?

Thanks, just wanted to make sure it wasn't some other sensor. As I know later years got a MAP.

I don't think the cruise control is on, but doesn't that turn off when you hit the brakes?

I've haven't messed with any cables. I'm gonna check some sensors and do the resets and see what that does.

I'm sorry, I wasn't clear. The part that is touching the screw is the throttle plate. As that plate stops at the screw. So, it is in the closed position every time I have looked under the hood, as I could see it being a problem if the throttle wasn't shutting all the way.

On the high rev at start up. When I crank it and it starts. The revs go way up like to 2200, then the next several seconds it drops off down to the 1000 or where ever on a cold start. And if it is warm, it will drop off to the 600 or 700. I forget where is idles at on warm. Then it idles fine, and I put it in gear and drive around with no other problems.

Only 2 times on a warm start did the high revs drop down, then start "hunting" by the revs going up and down like 700 to 1000, back to 700 then to 1000. Something like that, but I forget the exact RPM numbers.

 

[Nick90vetteguy]

AH HA! You my friend may have a cold start problem! if not its the throttle

 

[joshwilson3]

AH HA! You my friend may have a cold start problem! if not its the throttle

It does it on both cold and warm start up. The only 2 times of the "hunting" start up idle till I put it in gear happened at a warm start up.

Tommorrow I'll do some sensor checks if I can, and do those resets and see what that does after a while.

Though I'll probably look at the IAC as after doing some reading. I think the IAC is responsible for the start up flare. So maybe it could be sticking or something and letting if flare too high.

Is there any checks I could do on the Coolant Temp Sensor? As I believe that can cause some problems.

 

[Nick90vetteguy]

as far as the coolant sensor i dont think that'll be a problem, but if u take it out remember coolant does leak out, if ur lookin to take it out, stick a tampon in the hole no joke lol it works had to use it to put my headers on lol, is your EFI sensor like stuck when you rev the motor? its on the opposite side of the TB it looks like a little trigger mine got stuck sometimes because I actually over tightened the TB in place. I think 9 times outta 10 it'll be your throttle linkage thats just me.

 

[joshwilson3]

as far as the coolant sensor i dont think that'll be a problem, but if u take it out remember coolant does leak out, if ur lookin to take it out, stick a tampon in the hole no joke lol it works had to use it to put my headers on lol, is your EFI sensor like stuck when you rev the motor? its on the opposite side of the TB it looks like a little trigger mine got stuck sometimes because I actually over tightened the TB in place. I think 9 times outta 10 it'll be your throttle linkage thats just me.

I think I've read of people checking the coolant temp sensor's resistance or something at different engine temps. But if that sensor has nothing to do with start up, then I'd look elsewhere.

Do you have a part name for the "EFI sensor"? I'm not sure what that is. But I say I could check it to see if it is sticking when I rev the engine.

 

[boomdriver]

On my 89, it was down for a month or so. I had to replace a rear hub, ujoints, power steering pressure hose, air pump check valve, and spark plug wires.

After that, I went to start it. But turned the key and nothing happened. Must have been the VATS thing I guess as I came back out an hour later and it started up.

But since I had started it back up, I've noticed a high RPM spike on a cold and warm start up. On start up, the RPM's shoot up, then take several seconds to fall back down. Before I realized what was going on, on a warm start up I put it in gear fast before the RPM's dropped down. And it kicked into gear hard.

I don't notice anything odd after it is in gear. Just on a start up, but more noticable on a warm start up. I see the RPM's shoot up to 2200 or so, but also have seen 4400. But that could be an error on the digital dash.

A couple of times on a warm start up, the RPM's hunted up and down till I put it in gear.

Last year, I pulled the throttle body to clean it. The IAC looked new. I reset the TPS and checked it to make sure it didn't have any dead spots. Replaced the PVC valve and 3-way check valve. Fuel filter has been replaced as well as spark plugs.

find air leak FIRST,. then move on to setting things. according to spec.

 

[joshwilson3]

find air leak FIRST,. then move on to setting things. according to spec.

Any tips on finding air leaks? I think I recall something of spray intake cleaner on the hoses, and if the engine stumbles. Then that is where the air is leaking?

Can an air leak only be noticable on start up? Just strange an air leak would all of a sudden happen. I went over everything to make sure I didn't unplug anything.

 

[TedC]

1. Did you reset the ECM?
2. Did you reset the IAC after resetting the ECM?
At startup, the ECM drives the IAC to move 160 steps. Normal hot idle IAC steps is around 20 for reference. ECM then uses targeted idle speeds based on several input variables (resistance in the coolant sensor is one of them). Even if the coolant sensor was reporting resistance for 0*F on a 80*F day, the ECM never targets the high level of RPMs being reported. However, if the IAC is sticking, the ECM may open it to 160 steps, yet it can't close fast enough when instructed by the ECM. Having scanner data would really help isolate this. I've given up on cleaning IACs, I replace them when rebuilding/cleaning throttle bodies.

 

[Nick90vetteguy]

Do you have a part name for the "EFI sensor"? I'm not sure what that is. But I say I could check it to see if it is sticking when I rev the engine.[/QUOTE]

As far as a part name i don't know, I do know that sometimes people have a tendency to over tighten their EFI a little to tight on the TB and sometimes will get stuck, happened to me. If you want a more professional opinion, take it to aamco let them do a quick diagnostic

 

[Nick90vetteguy]

Any tips on finding air leaks? I think I recall something of spray intake cleaner on the hoses, and if the engine stumbles. Then that is where the air is leaking?

Can an air leak only be noticable on start up? Just strange an air leak would all of a sudden happen. I went over everything to make sure I didn't unplug anything.

I never heard of this, if your looking for a leak, you might as well look for a busted gasket while your at it ;squint: Tell ya what, go open your hood on your car, take off that goofy black intake hose and just let the throttle show, start the car(It will be loud) then get yourself a beer in the mean time let it idle for a bit and watch closely what happens with the throttle and listen to any "Bogs" if your throttle closes at anytime of that idle period then your problem might be a dead cylinder ie, no compression in a cylinder if it is check the spark plugs. BUT thats your worse case senerio

 

[boomdriver]

Any tips on finding air leaks? I think I recall something of spray intake cleaner on the hoses, and if the engine stumbles. Then that is where the air is leaking?

Can an air leak only be noticable on start up? Just strange an air leak would all of a sudden happen. I went over everything to make sure I didn't unplug anything.

I don;t have any short cuts on finding air leaks...I've always had to use visual, sound and function as a guide.

There WAS a guy that used starting fluid to spray around the engine to see what caused the engine to rev....thats not reccomended by anyone. Some say that WD40 and/or carb cleaner will do the same thing, but its messy at best and may do more harm to the plastics and rubber.

Various gaskets and seals can fail anytime. Lack of use, drying out, overuse, AGE... Who knows. It just happens. I woke up to a leaking head gasket last year. No real reason why, No overheating, no freezing. It just failed.

A leak thats only noticable at start up is possible...perhaps when warm the leak is not as bad OR the cold condition allows the leak.That says something, the fun is in trying to figure out what exactly that is...

If you are pretty sure there is no air leak, based on the symptoms going away when warm or as you drive as things return to normal, it could be as simple as a sticky IAC valve.Look at what happens during the cold start cycle or the starting cycle alone. Cold start valves are just injectors, so those can get dirty too.. Also, something that I did not know until reading the FSM is that the ECM does have a "learning" system that records operation and uses that to help manage the engine in the future. If your ECM or battery was disconnected or the car was parked for awhile, it may just need to be driven so that it can "remember" how to run. Your spikes of 3000 rpm are kinda radical IMHO, but sometimes the simple answer is the best.
If the IAC is suspect, Like Ted said, just replace it. They're more trouble to adjust than it is to buy a new one.

Cold starts without throttle and allowing the engine to do as it wants is common to have cold idle go to 1000 then settle down. Mine fires off and drifts toward 900-1000 then settles back to around 750-800 then finally 600-650 as the temp passes 150*. When its warmed it fires off a little lower but it always jumps up a bit as it lites off...then settles as temps stabilize. The additional fuel for start-up can cause the initial rpm to wander for a couple seconds.
Good luck with it...
Go drive it 10 miles at normal temps and see how it acts afterwards.