Hi All,

I have a 93 C4 LT1 which stalls in gear when in the closed loop. In the open loop when started from cold the car idles at around 900-1000rpm and runs great. In the closed loop the car idles around 600rpm and the idles stumbles and eventually stalls. The car has been in and out of the mechanic's garage with new sparks, leads, fuel filter, injectors, o2 sensors and coolant temperature sensor. The problem persists- my question is this, is there a way to have the car stay in the open loop? I know this is not the best idea but as a short term fix it would be great.

Any help would be greatly appreciated!

Thank you.

 

Unplugging O2 sensor will keep it in open loop and ses light on.

 

Get a new mechanic maybe one that lives and breaths corvette. There is no Corvette shop in Ct that could not fix your problem. Throwing parts at this beast will only keep the after market parts store rich.

 

Agree

 

When you Corvette goes from "Open Loop" to "Closed Loop" it starts to rely on the information coming into the ECM from multiple sensors. One of your sensors is not doing it's job, careful trouble shooting will identify it for you.

The O2 sensor is important but no more than the engine's current temperature and the engines Intake Air management system which on your car is a M*** Air Pressure sensor as well as a multitude of other devices. If your car has a problem under load is different than it just won't go into gear without stalling. Is it an automatic or manual transmission? How does it stall? My MAP sensor started failing slowly and it was not obvious as I thought I had bad fuel or a dirty injector. Under the biggest loads it started causing hiccups before it failed.

You need to hook it up to a scanner and see what is taking place during and after the transition, that should identify the problem. I would normally say that it sounds like you have a bad O2 sensor as clearly would mess your idle up. Something giving the incorrect engine temperature like a bad Coolant Temperature Sensor. I agree that you are now wasting a lot of money which makes the mechanic happy and the parts people rich. Keeping your car in Open Loop will probably kill your catalytic converters as it tends to run rich in that mode.

It is also very possible that your mechanic could have put a defective part on the car which won't fix anything. I really think that a lot of mechanics are intimidated by Corvettes, they seem hesitant to try to diagnose something. They get so fixated on the big picture (Cool Car) they forget how to find the needles in the haystack.

My Buick was running fine and then it started to have trouble under loads, like going up a hill or pulling a trailer. I bought and installed a MAP sensor and the problem went away. My OBD 2 scanner said the MAP had seen "a higher than normal voltage" which pointed me that direction. I looked at RockAuto and they have the original equipment AC Delco for your car for $40.79. It might be something as simple as that or there are other possibilities that can be explored.

I wish you the very best in solving this little dilemma!

Best Regards,
Chris

 

Hi All,
Thank you for your replies, much appreciated! The issue is I live in London, UK and it is very difficult to find good American car mechanics- I know a little bit myself but am very intimidated on actually working on my cars!

I had thought of the MAP- the way the car stalls is strange, it only does it in the closed loop when warm, in fact the car only tends to go into the closed loop once the car has been driven, switched off and restarted. The idle drops and you can hear the engine compensating to keep it going. Sometimes it will just stall, other times it will stumble intermittently. Performance is strong though and there appear to be no other mechanical issues. To make matters more difficult, no codes have ever appeared...

Would the TPS cause such issues?

Once again, thank you for your help!

 

The UK does make it more of a challenge. Factory Manuals and a OBD1 scanner are required. How long has car been in Europe? Is EXP on rpo list on SPID label on console door?

 

Hi Kevova,
Thank you for your reply.
It does make it more difficult unfortunately! The car has been in Europe since 1994. I have been using the paperclip method to check codes and to see if the car is in open/closed loop but I agree a scanner would be useful.
I am going to put a new MAP in and then start looking at the IAC. Will also try and find a good scanner- any recommendations please?
Thanks!

 

Since it's been there so long I expect everything was changed for engine to run correctly on available fuel. Scanner snap on MT2500 with GM cartridges , Tech1 was factory scanner with atleast mass storage cartridge undated to atleast 1994 additional Y body cartridges are a plus. There are some datalogging programs for laptops which may work. IDK Some software doesn't always work in different countries. American car club may help with resources, car show season is a good time to mingle.

 

Good day....how many miles on your Corvette? I also have a 93 that had similar ruff idle once the engine got to about 150 degrees, closed loop. Has the water pump or opti ever been replaced? Hope you find what causing your issues these thing can drive you crazy!

 

Hi, mine has 97k miles on it and the water pump and Opti were replaced about 5 years ago or 10k miles ago. These issues are so frustrating! I'm thinking the car is over fuelling and running rich but cant figure out the source of the problem.

 

You could check the fuel pressure at the injector rail, should hold pressure for 5 minutes or more. If pressure falls off when engine is turned off could be a leaking injector! Also take a small wooden dowel, 18 inch long and place on each injector body, place the other end close to your ear, you should hear a very steady opening and closing of each injector.

 

It's a good idea to check for fuel in the vacuum line to your fuel pressure regulator. If so then it's bad and will cause drivability issues.