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Miss and surge once warmed up

Parrothead

Well-known member
Joined
Jul 23, 2003
Messages
63
Location
Lakewood, Washington, United S
Corvette
Quazar Blue 92
Hey all, My 92, automatic, has a problem once it warms up. Within a few minutes of reaching 197 degrees, I start to experience a slight miss. It starts with a small drop in RPM and then recovers. It happen across all RPM speeds, in any gear or in park. If I continue to drive or just sit and idle it gets progressively worse. As it worsens it starts to surge above where the throttle is at or idle if idling. The idle speed also begins to raise up to and above 1000 rpm. Once this problem starts It has to be shut off and sit until totally cold or it continues.

When I drive to work, 10 minute drive, if I try to leave any sooner than about 4 hours it starts missing right away. If parked more than 4 hours it starts occurring once warmed back up.

From reading other threads this is what I've tried so far:

There are no codes
Checked fuel injector ohms both cold and hot, 13 ohms +- .3 on all when cold, 12.5 +- .3 on all when hot
Disconnected O2 sensors and problem still occurs (no check engine light when they are disconnected, Thought that was strange)

The ECM and O2 sensors were replaced about a year ago, no problems until a couple of weeks ago.

Any idea's would be greatly appreciated.................
 

boomdriver

Well-known member
Joined
Dec 18, 2008
Messages
1,888
Location
texas
Corvette
87 z-51
Surging is often a sign or symptom of either a vac leak OR bad fuel injectors.

Ohms testing is only part of the deal....all that tells you is that the inj coil is within spec...the weaker it gets the slower and worse the inj performance will be. Dirty inj or clogged inj have a similar neg effect. They will drip fuel instead of spray. Wet fuel does not burn as well and confuses the o2 sensor which tries desparately to adjust the fuel pulse to compensate...and a cycle begins.

Your eng control should make the shift to closed loop operation AFTER a specific time/temp has passed, so the o2 sensor is not part of the plan until its warmed and run for X amount of time so d/c the o2 may not set a code YET, because the ecm is not looking for it YET. It will once the time/temp/run has met the min for the shift to CL operation. At some point the ECM will want to shift but with no o2 input it can;t....thats when a code sets. Thats basically what CL operation is....the o2 circuit is now closed (when hot) and delivering voltage to the ECM for fuel mngt.

Then again, your beloved Opti is always good for taking the blame of random acts of poor performance. Since the Opti works by way of the dark arts, witch craft, voo-doo, the occult, and no one can fix them, the cost of replacement demands that the other possible sources of trouble be investigated before running down to the OptiSpark store to get a new one......If you google animal sacrifice and optispark you WILL likely get some results...:ugh

I'd start with a very complete check of all vac lines fittings and seals. Then move on to spark plug condition. One or more will reflect the misfire and offer clues to whats going on in that cylinder and why.
Last, look carefully at the wire harness on the firewall to the inj rails. Its known to get brittle and break the insulating material and get cross signals to the inj from other things.
Don't walk past the simple things...fuel pressure, filter, air filter & duct system.
 

Parrothead

Well-known member
Joined
Jul 23, 2003
Messages
63
Location
Lakewood, Washington, United S
Corvette
Quazar Blue 92
Boomdriver,

Thanks for the reply,
I did did another visual inspection and don't know how I missed these rotted vacuum hoses :mad. (I am going to try and post pics) They are on the driver side under the fuel rail. Unfortunately they didn't fix the problem. Just for curiosity what are they for? Is there a way to test the fuel injectors other than the resistant checks? I replaced the Opti cap with a vented MSD one 3 or 4 years ago and am trying not to think about the Opti, (That was an entertaining job). ;LOL

IMG_0875.jpg

Does anyone know where this line leads? It goes behind the water pump and after that I can't see.
IMG_0876.jpg


IMG_0877.jpg
 

aklim

Well-known member
Joined
Feb 15, 2009
Messages
324
Location
Hartford, WI
Corvette
91 White Coupe
When I drive to work, 10 minute drive, if I try to leave any sooner than about 4 hours it starts missing right away. If parked more than 4 hours it starts occurring once warmed back up.

From reading other threads this is what I've tried so far:

There are no codes
Checked fuel injector ohms both cold and hot, 13 ohms +- .3 on all when cold, 12.5 +- .3 on all when hot
Disconnected O2 sensors and problem still occurs (no check engine light when they are disconnected, Thought that was strange)

The ECM and O2 sensors were replaced about a year ago, no problems until a couple of weeks ago.

Any idea's would be greatly appreciated.................

Sounds to me like it hits trouble or at least the trouble is exposed during closed loop.

Never bothered to ohm the injectors. Worthless as far as I am concerned. I send them out every few years for testing and cleaning. No, I don't waste time with "cleaner in a can" or snake oils, voodoo or virgin sacrifices. Although a virgin sacrifice might work, you never know but I can't find a legal aged virgin.

What readings are the O2 sensors giving? Are they moving up and down quickly or perhaps they are getting lazy? I don't know about your car but do you have pre cat and post cat sensors?
 

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