Greengurkan

Hi,


I got a Z28 1991 manual with a 5.0 TPI.


In cold start and open loop it runs fine but after a 10-15 min drive when she goes into closed loop, then the idle starts going between 500-800 RPM and also when driven with a little throttle the same result happens but at a higher RPM of course, hard to drive too. If you give her more gas you can feel it's not the same performance as ion open loop.



Since she runs good in open loop so I get a little confused what's causing this problem which happens only in closed loop.



I did a test today when this happend, I stopped the engine, pulled away the temp sensor connector for the ECM and started her up.

No problems since she was running in open loop.



I checked for Error codes but there aren't any.



Like 400 miles ago every ignition part was replaced incl. injectors.


Need help!!



Thanks!
/Magnus

Go Blue

Have you tried spraying brake clean around the intake manifold and checking for vacuum leaks after the engine is warmed up? It almost seems like 1 or more sensors have a faulty reading but they aren't out of the "OK" range enough to set an error code. Tuned Performance is a great person to reach out to and ask this question to. He's great at diagnosing driveability issues!

Tuned Performance

iTrader: (92)

The only thing I can think of is a silicone poisoned oxygen sensor.

Pro

No error codes when you disconnected the temp sensor?

Tuned Performance

iTrader: (92)

https://www.thirdgen.org/forums/dfi-...open-loop.html
You should have got a code 15

Greengurkan

Hi, and thanks for the replies!


Update,



Yes, after I disconnected the temp sensor I got an Error code 15 so I connected it back again.



Yesterday I changed the o2 sensor to an ACDelco but the same idle sourge occured when the engine got warm. Also the engine shakes a little when the car gets into closed loop, you can fell it when it happens and you loose enginepower as well. It's hard to keep the engine at idle, it's surguing between 500-1000 rpm and it takes like a second, up-down, up-down.......



I stopped for a while and disconnected the tempsensor again to make the car go into open loop but the same sourging occured so I guess it was still in closed loop.

I parked the car for an hour, disconnected the o2 sensor and drove home again, this time nothing happend except some sourging around 1500 rpm.


I have a AC that works except the it's not blowing any cold air. I think the system is empty, can the AC have something to do with this? Is the AC connected to the ECM?


I read that this engines ar working under pretty high temperature. I measured the ohm on the temp-sensor and translated it to app. 217F, I also measured the ohm on the temp-sensor for the gauge and translated that to app. 215 F.


I will check for vaccum leaks.


I'm just confused when the engine runs pretty fine in open loop but goes bad in closed loop......


//Magnus

Go Blue

The AC fan and compressor are controlled by the ECM in terms of the ECM monitoring the pressure sensors to make sure that the R12 or R134a isn't too low or there isn't too much pressure in the system. I don't see that being your issue though. When the Oxygen sensor was disconnected did you get a check engine light for that? The temperatures that you are getting are normal and your fan should be running at that point and the thermostat should be open as well. I'm wondering if you're getting a vacuum leak when the engine warms up as that wouldn't necessarily cause a check engine light to come on and would cause surging and a rough idle. When your engine warms up and goes into closed loop, try spraying brake cleaner around the throttle body near the intake plenum and at the base of the intake manifold. If the RPMs change or the engine smoothes out or dies then you have found a vacuum leak and need to replace that gasket.

91RS5speed

If the AC is in the off position it shouldn't be feeding any adjustment data to the ECM.

Greengurkan

I dind't get any a check engine light when I disconnected the o2 sensor.

I even checked with a metal wire on the obd connector but it was only 12 three times.

I havnt' had any overheating problem and I know the fan is working, I flushed the system last summer. I will check fot vaccum leaks, metal expands when it gets warm so that can be something.

I'm also wondering if the ECM is sending can send a bad signal in closed loop to the ICV so that it openes and closes all the time? Can the ICV be bad as well?


The car was running just fine last summer and started after being stored for almost 5 months in a cold garage with a temp of app. 40F. Well, I started the engine up once a month.

Tuned Performance

iTrader: (92)

Usually if a iac is bad it sticks in a position . Resulting in a low or high idle.im really not sure why in closed loop your having a surge. You might try wrapping on the ecm case with a screw driver handle when the engine is warming up and see if the surging occurs.

Go Blue