I have a 98 Bi-fuel cavalier, it burns fuel and CNG (compressed natural gas). Using this site I swapped a manual trans into my car, the swap went great, drives fantastic, huge upgrade.

However, I put the 5 speed ecm from the donor car into my car and now have an issue with running WAY to rich on CNG. I have determined with research that the CNG ecm has two different sets of timing curves in it, one for gas, one for CNG. Since I put in the 5 speed ecm it will not run correctly on CNG, I HAVE to swap my 3 speed auto, CNG ecm BACK in so the CNG works correctly.

My question is why is a computer flash or ecm swap required for a manual trans swap? I have scoured the site with no luck, my car ran with the original CNG ecm in it but I did not drive it with the manual trans and auto ecm. Truthfully, I only swapped ecm's because everyone said to do it, but I cant run it what so ever, it smells terrible out the exhaust and surges on the freeway. I HAVE to have functioning CNG, its the whole point of the car, I have to run the original, alternative fuels ecm, that is programmed for a auto.

By the way, my vehicle is emissions exempt because it is a factory CNG car, SO CHECK ENGINE LIGHTS MEAN NOTHING TO ME, if it will operate fine on the auto ecm, great. I will obviously swap ecm's tomorrow to see if it will operate correctly, but would like to know the technical data differences between the two computers if possible.

Thanks for any insights.

Mike

I should also note, they never built a cng cavalier with a manual trans, so a flash is not possible.

swap back to the ecm that is tuned for cng/gas. that way your engine will run fine. the only thing is your cel will be on because of codes for the auto trans that is not there. to me that would be a better idea than the ruining the engine.

I swapped the oem ecm back in, seems to run a bit better. Drove it home on the highway about 15 miles, no check engine lights yet.

Can u check your cng gas door for a part number? I know alot of people want it lol

K, got the ecm swapped in on Thursday, topped of the fluids after rebuilding the top end and set off for canada. I went about 3k miles in 11 days, just got back.

The trans slipping code came on the first time after a thousand miles. The code appears when I "horse" the car out of gear. Best way to explain it is I am not smooth in my transition to much or to little throttle between shifts, it throws the code. Other than that no issues, runs a little odd on startup occasionally but who knows, not really bugging me.

However, with 4 people in the car, a trailer, backpacking gear and suitcases, I smoked the bottom end somewhere in Washington. On the way back I did a 3 AM straight 40 weight oil change in a 7 eleven parking lot since the oil pressure idiot light was coming on at idle, than I ran out of cng in Idaho since they have no cng pumps which I planned on, however I did not plan on also losing my regular fuel pump that has maybe 1500 miles on it, also I think I plugged the cat since it reeks like rotten eggs up big hills, then the brand new clutch that was with my trans started to slip ever so slightly.

I seriously love this little car, I beat the ever living @!#$ out of it, it makes me money on the fuel savings, and works like a horse when I tell it to. In hindsight having the motor on the floor a few days before we went may have been a little ambitious, but the thing ran like a raped ape, for awhile anyways.

Time to research a mild cam, roller rockers, a header, a quiet exhaust, and a centerforce clutch before we go climb Mt. Whitney in August, I need just 10-15 hp to get this thing to pull my trailer how I want and I will be set.

I do believe you can turn off the auto trans stuff via HPTuners.

You can forsure turn off the MIL (CEL) codes for all the auto trans, so they won't trip your check engine light anymore.

Pretty sure you can straight disable the auto trans info tho in an auto computer, basically making it a manual trans computer.

Find someone local who has and knows how to properly use HPTuners and get them to flash your computer if you want, it will cost you $100 for the credits plus whatever the person will charge for their time.

mike demco wrote:My question is why is a computer flash or ecm swap required for a manual trans swap?

To give you the higher idle rpm that complements a manual transmission.. manuals idle around 900 rpm while automatics idle at 650 rpm. Also to get rid of the auto transmission programming and codes associated with it.. thus the check engine light.

I don't know if you're limited to the 4k rpm limit which you are with an auto car out of gear... since you have no gear selector.. I never really thought about how the PCM knows which gear you're in to limit you.. but now that I've said that, I'm going to crack a service manual and figure it out.

Mr lenko from my experience of running an auto ecm on my manual it does not limit your rpms just throws the trans codes for the shift solenoids.

Ah of course.. makes perfect sense. That's how it knows what gear you're in.. the solenoids pick up. Duh. It's Friday night, and it's (clearly) been a long week.

Time for beer!