Help! Idles fine until warms up then seems to starve for air

[bmanvette]

My 91 ran fine when put away last year except for slight stumble in the idle that I believe to be dirty/faulty injectors. Now as I get ready to fire it up and work on it, it seems to starve for air after a few minutes running. Sounds fine when on initial "high idle", but once it tries to settle in a lower idle, it stumbles and dies. Awful rich odor from exhaust as well. Enough to burn your eyes. I think it has something to do with warming up?

Any ideas?

 

[boomdriver]

"not enough air" is the same as too much fuel.

Are there any codes showing up? if so, they will tell you where to start looking. If not, then try to keep it running a bit longer so the ECM can set a code and display it.

So, unless there is a birds nest in the air intake duct to the throttlebody or a rag stuffed in there, lets look at the other side of this coin...

Too much fuel can be from what you already know to be a problem, dirty injectors. Fuel inj do not wear out, they just get dirty. They can stick open as well as be blocked, and they can short out and stay open. I have had this happen and had one bank pour raw gas in the left bank to the point that I had an extra 2 qts in the oil...This did not allow it to start, but while trying to figure it out I was powering the shorted injectors and fuel pump....In early L-98 C4's when one inj fails or shorts, that whole side does too. They are "bank fired" meaning the 4 on each side fire together.
To evaluate this, check fuel pressure, and perform a leak-down test to see if the injectors are holding pressure,. might also look at some spark plugs to see if they are dark and wet with raw fuel.

The TPS could be bad, as well as the MAF sensor that tells the ECM how much fuel to send to the injectors. If yours is MAP, not MAF the same end result happens from a bad sensor. I do not know if the 91 has a temp sensor for cold starts. A false signal to the ECM triggering extra fuel is possible. Thats where stored codes are so helpful.

The only other thing would be weak spark that was not burning the fuel, but that usually comes with a no start, no fire symptom.

Find a fuel pressure gauge, a noid lite, and a plug wrench and get busy. A real factory service manual (not a useless Haynes or Chitlens manual) will also help guide you. Refered to as a FSM, ebay has them used for $75 for the 2 book set. The FSM is priceless when trying to diagnose things like this.

 

[vetteoz]

. fine when on initial "high idle", but once it tries to settle in a lower idle, it stumbles and dies. Awful rich odor
I think it has something to do with warming up?
Any ideas?

As the major difference between open ( cold ) and closed loop ( warm ) is the O2 sensor coming into operation and controlling the fueling
I would be checking that 1st .Pull the codes
C4 Corvette Diagnostic Code Recovery Techniques

and see if Code 13 is showing

 

[ex-x-fire]

Get a multimeter & a cooking thermometer, remove your coolant temp sensor & test it for resistence at different temps in water thats being heated up. Check it at room temp to 210*. Google "GM coolant temp sensor resistance" for the values.

 

[vetteoz]

Get a multimeter & a cooking thermometer, remove your coolant temp sensor & test it for resistence at different temps in water thats being heated up.

Engine Coolant Temperature Sensor.
185 Ohms @ 210F,
3400 Ohms @ 68F,
7,500 Ohms @ 39 F.

 

[bmanvette]

Thanks for the tips!

Thanks guys. Will get to work on this Saturday. Will pull the codes and see where it leads me.

Thanks again!

 

[aklim]

In early L-98 C4's when one inj fails or shorts, that whole side does too. They are "bank fired" meaning the 4 on each side fire together.

To evaluate this, check fuel pressure, and perform a leak-down test to see if the injectors are holding pressure,. might also look at some spark plugs to see if they are dark and wet with raw fuel.

If yours is MAP, not MAF the same end result happens from a bad sensor. I do not know if the 91 has a temp sensor for cold starts. A false signal to the ECM triggering extra fuel is possible. Thats where stored codes are so helpful.

Aren't all L98s bank fired?

I would think that since his injectors are dirty, he might as well get them sent out and tested so he can eliminate one variable. I tend to send mine out every few years or X miles so they can clean and test it.

91 is MAP because it is Speed Density so no MAF. Also, IIRC, after 89, they eliminated the cold start injector.

 

[G Winter]

Does it overheat at all ? if so you might want to check the cats.

Glenn

 

[aklim]

Don't remember about the stock setup but can't he simply disconnect the exhaust after the manifolds and vent to atmosphere? That way, after driving a while he can see if it is really having a cat blockage issue or not. Obviously he has to be careful to not idle too long lest he suffocate

 

[boomdriver]

Don't remember about the stock setup but can't he simply disconnect the exhaust after the manifolds and vent to atmosphere? That way, after driving a while he can see if it is really having a cat blockage issue or not. Obviously he has to be careful to not idle too long lest he suffocate

:L
yeah...that could be problematic..

The driver side is ok, but the passenger side has some goodies in the line of fire...blowing on or near the fuel line, starter solenoid wiring...knock sensor..

I know from a previous experience that a cat-block can sneek up pretty quick. I had one that happened so fast that the day I noticed something wrong, within <40 miles it was unable to build enough power to get past 30 mph. Limped to a shop where it was diagnosed.

The FSM says that you can pull the o2 sensor and screw in a gauge that reads low PSI...I think it was only 3 or 4 psi back pressure and thats considered to be plugged. The stock exhaust system creates very very little back pressure. I suppose when you;re looking for negative pressure near the heads to help scavenge, 2 or 3 positive PSI is a lot.

 

[bmanvette]

Up and running Update!

hey guys
Pulled the code....15....Coolant temperature sensor. Checked the connection and found corrosion. No other codes present. Cleaned up the connector and sensor pins. All is good. Idling like a charm. I can't thank you enough...."Vetteoz" you nailed it dude. I would never have thought something like that could affect it so severely.

I learned apparently that the computer runs the show!
Thanks again to all that assisted....

Craig

 

[vetteoz]

.Coolant temperature sensor..."Vetteoz" you nailed it dude.

It was ex x fire who suggested the CTS

I learned apparently that the computer runs the show!

The computer runs everything ; so anytime you have a problem pull the codes and you might get a answer straight up
http://www.corvetteactioncenter.com/forums/members/ex-x-fire.html

 

[bmanvette]

My mistake! You nailed it Ex X FIRE! CTS....Thank you!

Get a multimeter & a cooking thermometer, remove your coolant temp sensor & test it for resistence at different temps in water thats being heated up. Check it at room temp to 210*. Google "GM coolant temp sensor resistance" for the values.

My mistake! You nailed it Ex X FIRE! CTS....Thank you!

 

[ex-x-fire]

I had to break my arm to pat myself on the back.
Good job, glad you got running good.

 

[bmanvette]

The computer runs everything ; so anytime you have a problem pull the codes and you m

It was ex x fire who suggested the CTS


The computer runs everything ; so anytime you have a problem pull the codes and you might get a answer straight up

Will do. Fist time using the scan tool but realized it's invaluable. Thanks..

 

[vetteoz]

Fist time using the scan tool but realized it's invaluable.

Always check the codes 1st.
On your model ,ECM will also display "H" ( history ) codes; faults that have shown up in the past but are not currently present
A possible indicator for a intermittent problem

Be aware a code indicates a problem with that " system " ; not that the particular item itself is faulty.
A code for say , low O2 volts could indicate the O2 sensor itself is faulty but could also mean the wiring is bad or disconnected.
Both would give low volts on the O2 circuit back to the ECM and make the code show up.
The FSM has flow charts to follow to identify exactly where / what is wrong when a code presents

 

[bmanvette]

Thanks Vette oz

Yeah now that I've gotten familiar with the scan tool and service manual, should be able to narrow things down.

My first vette but looking forward to owning it for a long time.

BTW not sure what previous owner used but want to choose a decent oil and stick with it. '91 with 90K miles...Mobil 1?

 

[aklim]

Yeah now that I've gotten familiar with the scan tool and service manual, should be able to narrow things down.

My first vette but looking forward to owning it for a long time.

BTW not sure what previous owner used but want to choose a decent oil and stick with it. '91 with 90K miles...Mobil 1?

Mobil 1 is a pretty decent oil unless you have some special needs

 

[bmanvette]

Thanks aklim

Mobil 1 is a pretty decent oil unless you have some special needs

Sounds like a plan. I also noticed a recommendation from some users to contact Jon at FIC for injectors. Anyone got more info on how tocontact him. I had two injectors reading 11 and 8. Rest were about 15 ohms. Believe they are stock. For $200 i think I would just as soon change them all.

 

[aklim]

Sounds like a plan. I also noticed a recommendation from some users to contact Jon at FIC for injectors. Anyone got more info on how tocontact him. I had two injectors reading 11 and 8. Rest were about 15 ohms. Believe they are stock. For $200 i think I would just as soon change them all.

Good idea. CorvetteInjectors.com

Ohm tests are not much good when it comes to testing whether the spray pattern and volume is good. All they can tell you is that the coil has failed or not. I would recommend sending the set every few years. I just did mine. Sure, all the ohm tests came back good but the spray volume was all over including 1 of them that had a collapsed basket filter that was spraying way less. Hard to notice it since they don't suddenly fail but fail gradually.