I have had my 94 Formula for about a year and a half now. Ever since I got it, it has given me lousy gas mileage (between 13 and 16 mpg between city and highway), and had an inconsistant miss up top (only when railing on it) in non consistant gears. So far I have only put about 5,000 miles on it, but it is driven on almost a daily basis. I have changed the plugs, wires, 02 sensors, coil, 160 thermostat, LT4 knock module, and had the computer tuned. Still the lousy mileage, and miss remains. After scanning the car (numerous times), no codes were found either. It has always had good gas in it, and the fuel filter has been changed as well. With me still driving it almost daily (even though not long distances), and it not giving any codes, could it still be the opti? Or could it possibly be something else?

 

Doesn't sound like an opti problem. Are you getting engine codes? Check for exhaust leaks before the O2 sensor, especially near the back of the manifold where the bolts sometime break or back out.

 

Hmm. I'm about to go on Opti #3 in a few weeks. Sounds like an opti symptom. I lose my miss somewhat on the top end. Check for a good spark.

 

If it were Opti IMO it would have died by now. BUT I could be wrong.

 

Just a wild thought with poor gas mileage and skip at high rpm range could be a bad or broken valve spring how many miles are on the car?

 

What about an ignition control moduel?

 

With you running the msd ign you might want to check your coil also, ours started to develope "white" spots on it which is caused from heat and as the bandit said that icm can cause bad gas milage Our car had a problem of just shutting off! We replaced the msd coil and the icm and that seeming has fixed the problem and we also were getting poor gas mileage there at the end this weekend we just did the water pump,wires and plugs and man did that make a world of difference!Go to Shoeboxes web site and do the mod that will get that coil and icm away from the head and the heat!

 

The car has 75,600 miles on it currently. When ran on my buddies laptop, no codes were found, and everything was right where it was supposed to be, even when it missed up top. The exhaust was put on fairly recently, and again this has been a problem since I purchased the car a year and a half ago. The coil was new, although I have heard of some MSD products being faulty from the factory. A new opti will be put in the car in April, however I am just curious still if it really is the culprit.

 

My first thought about the fuel mileage is you drove for a year and half almost everyday but only put 5K on it. If you only make short trips without warm up, you could be doing a lot of your driving while it's still in open loop.

One quick test will tell you how it's burning in the cylinder and even point to a fuel or spark issue is an exhaust gas analysis. Some gases will go up due to running rich or lean etc....

The shop that can do that will also be able to hook up to an ignition scope and pinpoint any ignition issues. You can do a basic check for spark at all the plugs but that can be misleading.

The first thing I'd do is either have the injectors cleaned on the rail with something like I use from 3M or remove the injectors and have them cleaned and flow checked. Cars that sit for long periods (It may have sat before you got it since it only has about half the normal mileage) can develope some sort of fuel supply problem.

 

Most times when the car is driven, it is only for about 5-9 miles at a time, and usually it just makes it out of closed loop. At least once a week I drive it on the highway for at least 30 minutes as well. Last week I went through an entire tank with at least 85% of it being highway miles, and it got 17 mpg. That still seems to be a substancial amount less than it should. I will look into the injectors. Anyone else have any ideas? I would really hate for this thing not to end up being the opti.

 

If you believe the cause is anything in the ignition, the plugs would show evidence of that. When was that last time it had plugs and wires?

Rather than replacing the whole opti. Why don't you put a cap and rotor on it?

 

It's really tough to diagnose things on these cars, and the opti gets blamed for many more driveability and poor running conditions than it probably should. With that said it does deserve some of the bad rep that it gets.

In my opinion if you were experiencing an opti problem over this period of time it would have fully failed by now, or at the very least gotten substantially worse.

I'm about to start diagnosing some ignition problems on my car, and I can't even get it to run well enough to troubleshoot. Just think of this as a process of elimination. You have plugs, wires, opti, ICM. I am not a big fan or throwing parts at a car to try and fix the problem, especially with optis- I'm not broke but I'd prefer not to make a $250 guess. It would help if you could find a buddy with a similar car that you could "borrow" parts off of to diagnose. In order of difficulty to get to, I'd look at the ICM/coil, plugs, wires, then opti in that order.

 

The plugs and wires were changed about a year ago. The plugs that I had pulled out actually looked fine. No signs of running lean or too rich. Since I had bought the new plugs anyway, I went ahead and changed them, along with the wires. The coil was changed a month later, so all this has fairly been recent as far as the parts that are on it.

Injectors checked out fine as well. I might change the cap and rotor. You would really think something like this would be easier. It would help if it was consistant. I just got done railing on it about 30 minutes ago, and it ran fine with no problems. However I am confident if I were to open it up later it would break up on me again.

 

when i am crusing at a steady speed ie 70 on the interstate the rpms will go down about 300 then jump back up 300 over and over, this just started happening, could this be an opti problem? or what?

 

Go hi jack another thread.