Good morning. I am new to this forum. I just recently purchased a 94 red coupe and I am crushed to find out that after driving it home it is not the car that I thought it was at the time of purchase. It has 108,000 miles and it is an automatic.

When the vehicle goes into closed loop it idles very rough. It blips from 600-700 rpm to 1200 rpm. It's just all over the place. When the car is in open loop it runs like a top. I also have issues out driving around once the car is warm. It will stumble and hesitate when I give it some gas. It is not throwing any codes at this time.

We pulled the plug to the IAC when the engine was stumbling at idle. This leveled off the idle immediately, it stuck at 900 rpm and ran beautifully.

My neighbor is a Ford mechanic (I know, I know) and we have begun to troubleshoot the problem. We took off the TB and gave it a good cleaning. We also cleaned up the IAC and the MAF. The coolant was pretty low so we topped that off too. I checked the resistance of the injectors both hot and cold, they appear to all be in spec. I realize that doesn't mean much because that test has nothing to do with the actual spray or leaking of an injector.

I talked to a local GM dealer and they are pointing me towards a timing issue and discussed the possibility of the opti going bad. I am handy with tools, but I am far from being a gear-head. The last thing I want to do is start throwing parts at the problem hoping to fix it. I want to understand the car and follow a logical pattern of testing.

I have been spending all of my free time reading the posts in this forum looking for answers. This problem seems to be a common one, but the causes seems to vary greatly. I'm wondering if I should buy an ALDL usb cable and download some free software to log some real-time info.

Please, any help you guys can offer would be greatly appreciated. I'm a little heartbroken that my new purchase is now sitting in my garage broken. I'm very sorry for the long post.

 

If the engine runs well cold, but not warmed up, eliminates the Optispark Distributor as a problem. Timing also is eliminated as an issue.

You need to extract trouble codes from the PCM (Powertrain Control Module) to start to diagnose correctly.

Do a search in this forum for trouble codes and you will find tons of information.

If you find no current trouble codes, then report back and we will show you how to troubleshoot your problem.

 

Can I do the paperclip method on a 94, pins 4 & 12?

 

If you have 12 pin connector:

F E D C B A
G H J K L M

Connect "A" and "B".

These are the two top right terminals.

"12" should flash three times. Then any stored trouble codes will flash.

The trouble codes will flash three times, so you can write them down.

After all the trouble code numbers, "12" will flash again.

Hope this helps!

 

Torch, I want to start off by saying thank you for your help. It is much appreciated.

The 94 is an OBDI system with an OBDII connector (16 pin).

 

- What were the injector ohm readings?
- How do you know it's going into Closed Loop?
- Have you verified EGR operation?
- Is the digital temp gauge reading correctly? A bad coolant temp sensor can cause funny things.
- How old are the plug wires? As they heat up from being near the exhaust manifolds, they can break down.
- What's do the plugs look like? Gaps checked (and if so, what were they)?
- etc

Regarding timing...I had a 48k mile '90 L98 car that had this...ran fine when cold and then hesitation and surging when it entered closed loop until ~20* before it reached normal operating temp. Turned out to be a stretched timing chain. This one had me scratching my head and trouble-shooting for a long time.

 

- What were the injector ohm readings?
12.2 to 12.4 cold
13.4 to 13.6 hot

- How do you know it's going into Closed Loop?
I guess I don't for sure

- Have you verified EGR operation?
I have not, definitely something we need to look into

- Is the digital temp gauge reading correctly? A bad coolant temp sensor can cause funny things.
The temp gauge does appear to be reading correctly

- How old are the plug wires? As they heat up from being near the exhaust manifolds, they can break down.
The plug wires look like they are the originals. I will be replacing the plugs and wires shortly.

- What's do the plugs look like? Gaps checked (and if so, what were they)?
Haven't pulled them yet.


Thanks for the insight 96GS, I will look into these things.

 

Check the O2 sensor itself, maybe it is bad and is providing incorrect information to the computer? Do you have an air leak in the exhaust side where the O2 is located that could be confusing the O2 sensor?

If you can get a WB in the exhaust what are the readings in open loop and closed loop? The closed loop reading should be around 14.7 is that what you are getting?

 

How do you know when it is in open loop and then going into closed loop. Unless you are using a scanner you can't know this. If you were using a scanner then you could see a correlation when the problem starts. Other than that you are guessing because it goes into closed loop only after certain conditions are met and it can take different lengths of time.

With a scanner you can look at the O2 voltage swings, pulse width of the injectors, TPS voltage and fuel trim numbers (BLM counts). This will tell you if the fuel A/F is correct because in closed loop the O2s become the major input for A/F ratio.

Have you tried unplugging the MAF to see if it runs better?

Actually a very common fault of the opti is to act up when the engine heats up. It can also cause the idle to wonder. It will also cause bucking or hesitation in the higher gears under acceleration. Shifting down seems to smooth out the problem and cover it up.

 

Without hooking your car up to a scanner your just guessing....WW

 

You guys are correct, I don't know for sure that it is in closed loop. I am just going by what my mechanic buddy is telling me, I am in no way a mechanic. I will revise it by saying once the car is warmed up.

I'm trying to get my hands on a scan tool, but I am having difficulty since the 94 is an OBDI system with an OBDII connector.

I appreciate everyone's input, I will make sure I provide hard data and not just assumptions in order to better diagnose the problem. I will report back once I have better information.

Thank you for the help.

 

Go to Advance Auto or one of the other parts stores, Advance will scan your system for you and let you know if they see anything abnormal in the reading...If nothing else at least you will know if its going into closed loop..If your going to do your own work on these cars there are 2 things you will need ...A "Factory Service Manual" and a Scanner.."AutoXray" makes a good unit for personal use..........WW

This is what I have..
http://www.amazon.com/AutoXray-6000-...y+ez-scan+6000