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Discussion Starter · #1 · (Edited)
I took my 98 Cobra in to Apex Motorsports last week to get a Bassani X pipe put on and to first get a baseline Dyno run. After a couple of runs on the dyno the mechanics called me over and showed me some incredibly dissapointing numbers, the worst of which was an A/F mixture showing 14.2 across the line until 5700 rpms where the computer promptly dumped fuel into it and the ratio moved down into the low 11's. The mechanic said "You are scary lean" and said that he didn't reccomend another run so as to not harm the engine. We unplugged the computer for several minutes and tried one more run at which time it ran rich the entire run. One more try...back to it's former tricks...scary lean to 5700 and then scary rich. I don't even want to talk about the HP numbers...let's just say a Chevette might pull me right now!

My car is now in the hands of my trusted mechanic who has done all the maitinance on the car and is excellent with electronics. He is giving it a thorough going over and doing the necessary diagnostics.

Have any of you had this happen? If so, what was wrong and how did you resolve it? Could be any number of things...fuel filter, oxygen sensors, MAF...who knows.

Any feedback or suggestions would be a big help. I'm supposed to do my first open track event this weekend and can't run it like this.

Oh ya...my mechanic pulled the plugs and told me that they looked well within the normal range. They are the originals...just 31k on the car.

BTW, the X pipe sounds great. I have dynomax ultraflos welded in to the stock catback...nice combination.
 

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Scott98Cobra said:
I took my 98 Cobra in to Apex Motorsports last week to get a Bassani X pipe put on and to first get a baseline Dyno run. After a couple of runs on the dyno the mechanics called me over and showed me some incredibly dissapointing numbers, the worst of which was an A/F mixture showing 14.2 across the line until 5700 rpms where the computer promptly dumped fuel into it and the ratio moved down into the low 11's. The mechanic said "You are scary lean" and said that he didn't reccomend another run so as to not harm the engine. We unplugged the computer for several minutes and tried one more run at which time it ran rich the entire run. One more try...back to it's former tricks...scary lean to 5700 and then scary rich. I don't even want to talk about the HP numbers...let's just say a Chevette might pull me right now!

My car is now in the hands of my trusted mechanic who has done all the maitinance on the car and is excellent with electronics. He is giving it a thorough going over and doing the necessary diagnostics.

Have any of you had this happen? If so, what was wrong and how did you resolve it? Could be any number of things...fuel filter, oxygen sensors, MAF...who knows.

Any feedback or suggestions would be a big help. I'm supposed to do my first open track event this weekend and can't run it like this.

Oh ya...my mechanic pulled the plugs and told me that they looked well within the normal range. They are the originals...just 31k on the car.

BTW, the X pipe sounds great. I have dynomax ultraflos welded in to the stock catback...nice combination.
First, I assume that the wideband that they are using is not the problem as you said that your power levels are seriously down.

Also, assuming that your PCM isn't throwing any codes as evidenced by a steady MIL light, I'd start by doing the basics. Since your car has 31K, I'd do a basic tune-up (i.e. replace the *fuel filter*, spark plugs, etc.) before I even began to debug the issue at hand. Since you have an oiled-qauze air filter, I'd clean the mass-air meter element *carefully* because I've seen many times that oil from the filter contaminates the hot-wire which results in an overall lean condition at WOT. I usually use electrical contact cleaner for this but I believe that some people use brake cleaner without trouble. IMO, this is part of a regular tune-up if you're running a K&N or equivalent filter.

After the basics, I'd hook up a scan tool (OBDII scanner) and start poking around the LTrims and STrims on both banks, fuel rail pressure, MAF rate, etc..

It could be a problem with the O2's but more than likely not. However, for sake of arguement, let's see how the O2's could affect WOT operation.....let's say your O2's start to become less sensitive with age. As they become less capable of turning residual oxygen into voltage, they will cause the PCM to adapt (in normal part-throttle cruise situations) by increasing the LTrims in the negative direction (LEAN). Then as you go WOT, the open-loop fuel modifier, which is computed by what the PCM has "learned" during its adaptive strategies will be applied to your WOT a/f map causing your car to run lean.

Well, I can't diagnose your specific problem from here BUT I just thought that I'd share my thought process with you. Best of luck!

Mark
 

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Any intermittant problem should be caught by the OBD-II stuff (sometimes it IS useful). For your results to vary so widely from run to run I'd look for a mechanical cause, like a fuel pump about to die. Check fuel pressure, etc. A good scanner can tell you lots about what the engine is actually doing.

David
 

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Discussion Starter · #5 ·
Thank you for the feedback guys. As a follow up, I took the car in to my regular mechanic who has done all the maintenance, etc on the vehicle, except for the mods. He said the car checks out great, the spark plugs showed no signs of burning lean but did have some deposits on them. He indicated that these would not produce the results that the dyno shows.

He checked out the O2 sensors and the fuel injectors and they were fine. So...it appears that their equipment was not functioning correctly or something.
 

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Scott98Cobra:

There are too many unknowns in your account to zero in on anything or even conclude there actually is something wrong at this point. I'm really wondering where the wideband sensor was located. Before a cat or in the tailpipe?

What are the disappointing HP numbers exactly?

I can also tell you (cause I've got a '98 Cobra and I've seen its OEM tuning data) that A/F in the low 11s at high load and RPM is exactly where the factory tune wants it to be. It also indicates the fuel system is not fundamentally inadequate.

Something you can try:

Make sure the MAF sensing wires are free of filter oil residue. Disconnect the battery. Open a door or push the brake pedal for a minute to discharge the big capacitor in the Air Bag Controller and the capacitors inside the EEC. Those capacitors can preserve the EECs adaptive memory for quite a while if you haven't done something to discharge them.

Basically, you are clearing the EECs adaptive memory because it may contain compensation for a dirty MAF. Drive it for a bit afterward to let it re-learn before you dyno it again. Make sure the wideband sensor is before the cat so you know you're seeing the true A/F.
 

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Rick Evans said:
Scott98Cobra:

There are too many unknowns in your account to zero in on anything or even conclude there actually is something wrong at this point. I'm really wondering where the wideband sensor was located. Before a cat or in the tailpipe?

What are the disappointing HP numbers exactly?

I can also tell you (cause I've got a '98 Cobra and I've seen its OEM tuning data) that A/F in the low 11s at high load and RPM is exactly where the factory tune wants it to be. It also indicates the fuel system is not fundamentally inadequate.

Something you can try:

Make sure the MAF sensing wires are free of filter oil residue. Disconnect the battery. Open a door or push the brake pedal for a minute to discharge the big capacitor in the Air Bag Controller and the capacitors inside the EEC. Those capacitors can preserve the EECs adaptive memory for quite a while if you haven't done something to discharge them.

Basically, you are clearing the EECs adaptive memory because it may contain compensation for a dirty MAF. Drive it for a bit afterward to let it re-learn before you dyno it again. Make sure the wideband sensor is before the cat so you know you're seeing the true A/F.
Good advice! :cool:

Mark
 

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Discussion Starter · #8 ·
Thanks Rick for the helpful info. OK here's the really bad news. The HP was 239 on the first run, 242 on the second run and 252 on the third run. The third run was made after disconnecting the power for a few minutes to reset the computer.

The sensor for the A/F ratio was inserted in the tail pipe. The A?F ran at 14.2 at WOT from 3K to 5700K where it then dropped to 11.3 or so. This happened on the first two runs...on the third run it ran in the mid 11's across the power banc.

This was the first dyno run on the car ever and I am not familiar with the various setups or where the equipment is supposed to be connected for optimum results. I am just very dissappointed in the horsepower numbers. I did have the car tuned and had some diagnostics done as I mentioned and it definately runs stronger. It seems to me that the problems in the computer tuning are happening at WOT and not normal driving conditions. If you have any other suggestions as to questions I can ask the techs or other options to check out I would really appreciate it.

Thanks again!
 
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