vettl98

Hello Guys
my friend own a 1992 Corvette LT1
with auto
the car run cold very well nice all stock
but when she run after 2 or 4 minutes she start
to run not exact she start to shake at 1500 to 2000 rpm some missfire
after 2000 rpm she works nice and without problems
we change the Optispark to a new MSD the coil also and the injektors also
new the sparkplugs also new it will be a little bit better but the problem is still the same
the o2 sensors was change 1 year bevore and drive after 1000mils
the Ignition Wire are from Tayler 8mm looks new but i am not suer
i will change the Ignition Wire also and look what she will do
hase anyone the same problem ?
please let me know
thanks a lot
Jochen

Mohammed_z

check for codes !

96GS#007

The fact that this occurs after a couple minutes says that when the ECM goes into closed loop (using the o2 sensors for idle and part throttle fuel control), there is an exhaust mixture issue. You've replaced all the key components. However, there are two other systems that if faulty, will give you the symptoms you are experiencing...

- EGR system
- Secondary AIR (Air Injection Reaction) system

Of the above, the most typical failure is the EGR valve. I wouldn't be surprised if it's stuck. When this occurs, you'll get a low rpm misfire during closed loop.

The AIR system has two one way check valves. One above at the rear of the intake manifold (with piping running to the passenger side exhaust manifold) and one above the driver's side exhaust manifold. It's not unusual with age for these valves to fail. This is the less likely possibility.

Personally, I'd pull the EGR valve off the back of the intake and check it for proper operation per the Factory Service Manual.

C4GUY

I have same problem and now too I am looking to the EGR after trying just about everything else.

93Rubie

So it runs good cold, misfires after 2 to 4 minutes of driving. Do I have this correct? Does it ever do this hot or is it just in the "warm" area of operation??

I have chased a similar issue on a friends 94. I have more hunting to do, I can tell what it is NOT, on that car. His is perfect cold, but coolant temps between 100 and 140 in the RPM range you describe it backfires thru the intake. Once it hits closed loop at 140+ it is like someone flipped a switch runs perfect.

If you start it hot temps over 180+, it will do it only for the first minute or so then into closed loop and it is fine. I'm sure it is a mixture issue, I have yet to find what is causing it.

jaa1992

If you don't have the FSM try this to check it.
Take the EGR valve off the rear of the intake.
See if it is closed all the way
take a hand vaccum pump and apply vaccum to the vaccum port.
Does the valve open? Hold vaccum?
now release the vaccum
Did the valve close all the way?

I'll bet its gunked up (technical term). If it holds vaccum and it opens and closes you can clean it up. If not replacement time.

Good Luck!

vettl98

Hello Guys
thanks for so much information i will check all this weekend and let you know
the Vett runs normal only at the cold starting in the first time after 2 or 4 minutes drive
she starts with shaking and misfire only when i have slow driving 1000 rpm to 2000 rpm
when i shift to neutral she runs like new after shifting in the gears she make the same
also with the new Ignition Wire
so i will check the codes on the weekend and the EGR and Secondary AIR
thanks a lot for Helping
wfg
Jochen

tlmarks

I have a 92 that had a similar problem, after 2-3 months of checking things and finding nothing the check engine light came on. It was a code for the coolant temp sensor. I replaced it and every thing was good again. I assume the sensor was failing but not enough to set the code, and was messing up the a/f ratio's.

vettl98

Hello Guys
sorry for the late answer but the time you know
so i change the coolant temp sensor and the Vett runs great so i think i have the problem
i make a long test ride it is now cold in Austria and she ruins like new but when i came
to 198 degrees Water Temp the vett starts to make the same she shake from 1000 rpm to 2000 rpm so i look all what you told me so i find the EGR valve off the rear of the intake will not work so i change
the EGR and make a test ride like the same as bevore i check the
fuel pressure the complete time of the test ride and it is always 42 to 45 psi
so it must be ok
i speak with a god friend and he think maybe the knock sensor
this is the only one how start to check at 198 degrees so i order 2 new and we will see
or have you any other idea
thanks for helping
greetings from Austria
Jochen

desertmike1

I had simular issue with a 95. Between 1500-2000 RPM the engine would stumble and loose power. After 2000 RPM it ran great, it did this for about a year, I did the normal checks and continued coming up empty handed....

And then one day my coolent temp (anolog) starting swinging gradually between about 220-260f the following day I had steem!!!

Long story cut short... after doing a top end the engine runs like butter, the stumble is gone, not sure why! maybe when I had it all apart and cleaned/resealed?

maybe just the head gaskets themselves...

The one clue I kept missing was the unsteady anolog readings...I sort of ignored it because I kept reading statements about how inaccurate or unreliable these gauges are. But after the top end the gauge now reads steady, and about 5-15 deg higher then the Digital at cruise.

test your TPS, clean the MAF check your wires for arching etc. Just try to avoid throwing parts at it!

richauto

Hi 93Rubie,I had the same problem with my '94. After scanning all sensors and scoping the ignition,I found the problem to the injectors. Before replacing them I tried Lucas injector/cleaner and lube. To my surprise it worked. But to make sure I drove it for two tanks of gas. I have not had the problem . I will replace the injectors this winter. But I thought this might help. Good luck

vettl98

Thanks Guys
for your information
i have order new Knock sensor and new Temp sensors all other sensors are new
so i hope i will find the troublemaker
it need always time to get the parts so i let you know when i get the parts and
know more about
thanks for your help
greetings Jochen

WW7

Instead of just replacing parts randomly you need to hook it to a scanner and get someone who knows how to read a scanner watch as the car goes into closed loop. This will hopefully show where the problem is by giving readings that are off in a certain area, this should point you in the right direction...WW

Bud Harris

HI MY NAME IS BUD I have a 95 C4 convertable I have a similar problem, when i start off cold it has no power and coughs back, pick up speed slowley, after running1/2 to 3/4 mile it smothes out and runs great. once in a while excelerating it will cough. It is the LT-1 engine. By the way I didnt mention that I am a new member and this is my first VETT .

Repzard

^^^^ This , These car can suck you wallet dry if you keep throwing parts at it..Been there and done it.My 93 is just about brand new under the hood and wasted so much money to a point
Now my 97 SVT Mustang Cobra is the money pit, but its worth it when its a numbered car and rare to a point with color combo.