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Help! Surging idle speed after water pump replacement.

mark96

Member
Joined
Apr 5, 2010
Messages
13
Location
Bourbonnais, IL
Corvette
1996 Collector Edition Coupe
Hello - I just replaced the water pump, distributor cap & rotor, and serpentine belt on my '96 LT1, and now the idle speed is surging from 500 rpm up to 800 or 900, and then back down again. The idle speed is steady upon cold startup, and begins to surge once the engine is hot. I've checked and rechecked all the electrical connectors and vacuum lines. This is my first maintenance project on this, my first Corvette, so I'm a newbie for sure. Anybody else experienced this or can point me in the right direction?

Since the parts change-outs, I have no coolant leaks, and the engine is running consistently at about 195 degrees. I believe (but am not 100% positive) that after three cold/hot/cold cycles I've now got all the air out of the coolant system. The engine runs strong and smooth with no missing or hesitation during both normal driving and more vigorous acceleration.
 

boomdriver

Well-known member
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Dec 18, 2008
Messages
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texas
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87 z-51
Throttle position sensors sometimes cause idle surges, as does timing being off. Idle circuit may be in need of cleaning, or adjustment. Dirty injectors?

if you checked the vac lines, and elec connection, look at the throttle body real well and clean with some jet spray to wash any gunk out of air passages.
Run some GOOD inj cleaner thru and see what happens. Optis are known all over the world for being hard to deal with, and fragile when wet.

My L98 does that when I restart hot, surges up to 800 for 30 seconds to a minute then settles. I think it does this waiting for input from all the sensors to reach the ECM and cycle into normal run. If there are no codes for this behavior, then its not bad enough for the ecm to care,.just annoying for you.

You DO have to drive a certain amount of time/miles at 30 or over steady speed for some things to reset. Do that with good gas and inj cleaner for 20-30 miles and see if it improves. It should.

whenever we d/c certain systems,. we have to reteach the car how to run. Hopefully thats all this is.
 

Hib Halverson

Technical Writer for Internet & Print Media
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71 04 12 19
Without looking at the serial data stream with a can tester, this is a guess--but that the engine idles smoothly after a cold start but begins to surge once it warms, sounds like the engine cannot control idle once the system goes closed loop. Has the check engine light been on at all?

Double check to make sure you didn't break or pull off any vacuum hoses during the work you did on the engine.

If you don't find anything obvious, I'd consult the discussion in the 1996 Service Manual which covers unstable idle and surging.
 

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
Did you get the water pump temperature sensor hooked up when you got done with the work. The connector goes on the water pump on the left side of the pump when you are standing on the left side of the car.
 

mark96

Member
Joined
Apr 5, 2010
Messages
13
Location
Bourbonnais, IL
Corvette
1996 Collector Edition Coupe
Thanks for the responses. I do have the water pump temperature sensor connected, and I can't see any vacuum hoses or connectors loose or missing. It's very hard to see the vacuum hose on the bottom of the opti, though. I looked with an inspection mirror and it appears to be on OK. After browsing threads here on erratic idle speed, I took the IAC off and cleaned it, and also cleaned the gunk out of the throttle body intake ports. I got no improvement. Then I drove the car for about 30 miles of mixed driving - in town, on the interstate, country roads, acceleration, etc. I didn't drive it for 30 miles at steady speed, however. It ran smooth with plenty of power. Sitting at stoplights in drive, there is no surging of the idle speed. It stays steady at about 600 rpm. When I got home, I put it in park, and the surging returned.

One question I do have is about the coil wire from the coil to the opti - should it run over the top of the water pump, in front of it, or threaded through behind? I couldn't remember how it came off, and I couldn't find even one drawing in the GM service manual that showed the correct installation. I threaded it behind because it seemed to fit best, but I'm wondering if I might be getting some intermittent spark to ground, causing the idle to drop momentarily, then surge back up. What do you think?
 

John Robinson

Gone but not forgotten
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Messages
1,555
Location
Muncie, Indiana
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1993 Polo Green Coupe
Go out and start the car in the darkest place you can find (that is safe). Preferable away from surrounding noise. Lift the hood with engine running and look for a high voltage arc around the wires and listen for the high voltage discharge of voltage going to ground. I am just guessing but in my experience if a plug wire is next to metal on these high voltage ignition's they will short out through the wire insulation and go to ground. The same is true if you have a plug wire touching another plug wire. You might also try to spray the plug wires with a spray bottle of water to see if the idle changes. And take a close look at the ground wires on each side of the engine and also the ground wire bundle for the TPS,MAF,water temperature sensor, which is under a glob of tape just under the removable cover over the injectors. If that ground bundle is corroded redo it with good clean ground connections.
 

mark96

Member
Joined
Apr 5, 2010
Messages
13
Location
Bourbonnais, IL
Corvette
1996 Collector Edition Coupe
Again, thanks for the help! In answer to Hib's question, the check engine light has not been on at all. I did clean up the ground connections on either side of the engine last night, but I could not find the one under the right side fuel rail cover that the FSM shows bolted on the front near the coil. I also didn't see a ring-lugged ground wire or pigtail that wasn't connected. In other words, I could find no ground connected in that area, nor any unconnected ground wire(s) hanging loose in that area. Either I'm not looking in the right place, don't know what I'm looking for, or the ground is not there at all.

In any case, the surging has improved somewhat since driving it and cleaning up the grounds. It still surges, but not as much or as often. Since I haven't had any warning lights, and since the engine seems to idle fine during normal driving, the surging when in Park has become more of an annoyance than anything. Next up, I'll get under the car and inspect the vacuum hose connected to the opti to make sure it hasn't come loose, and, once the lousy weather improves, add some injector cleaner and try driving for 30 minutes at steady speed to see if I get any further improvement. I'll let you know how it goes.
 

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
The ground under the fuel rail cover on my 93 is in the wiring harness . When you take the cover off you will see were there are two wire bundles that come together into the big bundle that runs back towards the firewall . If you open up that connection were the two come together you will see a common ground bundle (I think 5 black ground wires) If that bundle is corroded at all it needs to be cleaned since those grounds go to the ECM and it reads the sensors they are connected to through those wires. As I understand it the ECM reads the ground wire voltage and then determines how to make adjustments to the engine. On the drivers side of the engine there are four grounds the two you can see fastened to the frame rails and two that are hidden behind the battery. One of those is a flat braided ground strap that goes to a stud bolt on the bell housing. That strap is the ground for the ecm and controls everything the ecm does. On that stud bolt are other grounds. Finally disconnect the battery and remove the wire harness plugs to the ecm and use a contact cleaner and clean the connections on the ecm and pigtails that plug into it for corrosion.
 

mark96

Member
Joined
Apr 5, 2010
Messages
13
Location
Bourbonnais, IL
Corvette
1996 Collector Edition Coupe
I just took it out on I57 to run for 30 minutes steady speed, and the Service Engine Soon light came on after about 15 minutes at 70 mph. Ran it past my local Autozone, and they checked the code for me: P0160 - O2 sensor, bank 2, sensor 2. They had a replacement, so I bought it and will put it on shortly. I understand from another thread here that the bank 2, sensor 2 is aft of the catalytic converter on the passenger side. Also from another thread, that it will take three start/run cycles to clear the SES light. I'll let you know what happens.
 

mark96

Member
Joined
Apr 5, 2010
Messages
13
Location
Bourbonnais, IL
Corvette
1996 Collector Edition Coupe
What happened is that I could NOT remove the old O2 sensor. Bank 2, sensor 2 is nearly impossible to access. I dropped the catalytic converter a couple inches to gain better access to the sensor, but it wouldn't budge, even after repeated soaking with PB Blaster, tapping lightly, etc, etc, etc. I don't have any flame retardant material to shield sensitive areas, so I didn't want to apply heat from a torch. Even if I had been able to remove the sensor, I'm not sure at all that I would have been able to access the connector. It's crammed into an impossibly small area and clipped in place. The other three O2 sensors are a breeze to get at compared to this one. Whoever designed that location for bank 2, sensor 2 was sadistic. It seems to me the only way to remove it is to drop the entire exhaust system, then hope I can disconnect the connector. I'm not one to give up easily, but I may have to take it to a repair shop this time.
 

boomdriver

Well-known member
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Dec 18, 2008
Messages
1,888
Location
texas
Corvette
87 z-51
What happened is that I could NOT remove the old O2 sensor. Bank 2, sensor 2 is nearly impossible to access. I dropped the catalytic converter a couple inches to gain better access to the sensor, but it wouldn't budge, even after repeated soaking with PB Blaster, tapping lightly, etc, etc, etc. I don't have any flame retardant material to shield sensitive areas, so I didn't want to apply heat from a torch. Even if I had been able to remove the sensor, I'm not sure at all that I would have been able to access the connector. It's crammed into an impossibly small area and clipped in place. The other three O2 sensors are a breeze to get at compared to this one. Whoever designed that location for bank 2, sensor 2 was sadistic. It seems to me the only way to remove it is to drop the entire exhaust system, then hope I can disconnect the connector. I'm not one to give up easily, but I may have to take it to a repair shop this time.

Yes, they can be stuborn...
This IS a shop job.
...and yes, it does require heat...lots of heat. It takes a oxy/acty torch to get the surrounding exhaust pipe cherry red before that sensor will unscrew. Then you probably need the special sensor wrench or if its up on a lift, sometimes a boxed end will break it loose..They'll have what is necessary.

I have an aftermarket front Y on mine and they aimed the bung at the block, so when mounted its just about exactly the worst possible angle to try to access the thing...

BTW...
cost me $40 at a small muffler shop to have them remove the POS and screw the new one in place.

Avoid the big chain store muffler shops(Midas)...they seem to think this is a huge deal and want to write a work order up and have their minimum charge that applies... can't install customer parts..etc..
bunch more than $40..
 

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
I removed the o2 sensor on my 93 by removing the inner fender and using a long 3/8 extension with a deep well socket and universal joint. Naturally I had made up my mind to remove it so I cut the wire off so I could get the socket on from the top. I tried to brake it loose by using a 1/2 brake over bar and could not budge it so I got my trusty piece of pipe still no luck. Out of desperation I put my lug nut air gun on it and let it hammer away until it came out. I then put the new one in from the bottom and used a wrench to snug it down. They really don't need all that much torque just enough to not have an exhaust leak. I don't know if this will help but I thought that others might like to no that on an LT1 you can get the right side out from the top
 

mark96

Member
Joined
Apr 5, 2010
Messages
13
Location
Bourbonnais, IL
Corvette
1996 Collector Edition Coupe
John - was that O2 sensor in front of the catalytic converter or behind it?
 

mark96

Member
Joined
Apr 5, 2010
Messages
13
Location
Bourbonnais, IL
Corvette
1996 Collector Edition Coupe
I haven't posted in quite a while, but I've since checked the EGR for movement with a vacuum change and it seems ok. Vacuum hoses have been double-checked and appear ok. I have a friend with a code reader, and we found no engine codes showing. One thing that I've noticed, though, is that if I quick-rev the engine up to 3k rpm or so a couple of times after putting it in back in Park, the hot engine will idle just fine for a while. Does this shed any additional light on what my idle problem might be? The front O2 sensors, maybe? With 50k miles, I still have the original O2 sensors, plugs and wires, as far as I know.
 

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