VSS involvement with fuel trims

**Greezmonkey** · 05-01-2020

I have transferred a 94 Buick Roadmaster drive train into a 93 Saturn Wagon. It is now rear wheel drive and a blast to drive. My question is, how does the VSS effect the fuel trims? I have installed an Abbott electronic ratio adaptor mostly for transmission shift improvement due to the different tire size and rear end ratio. The VSS output speed via the lap top data is about 23 mph faster than indicated speedo that is a calibrated Auto Meter. The first indication of VSS issues occurred when the injectors shut off when the computer VSS indicated approximately 110 mph. The drive train has about 135k miles on it and all components except injectors, knock sensors and map sensor have been replaced. Opti has new housing/bearing that I swapped in the original Mitsubishi head and matching disc. Old one worked fine but bearing was worn. Plug wires are a week old MSD 8.5 spiral core.
I have just over 2k miles on the project now with it being 95% great drivability. There is still a slight stumble at steady throttle slow or cruise hwy speeds that kicks lock up in and out. Little long but wanted to provide as much info initially as possible to determine VSS involvement with fuel mixtures. I do have data from driving trips. Thanks for your help.

**steveo** · 05-01-2020

the VSS doesn't affect the trims themselves but it does influence which BLM trim cell is selected. cell 16 is for a stopped/idling and cell 17 is for moving closed throttle/decel

the biggest affect the VSS has on how the engine runs is via IAC control. it has a speed follower, and some logic for coast down and decel stuff

it's common for the datalog speed to mismatch actual speed, there's a scalar for that if you want to fix it though

**sherlock9c1** · 05-01-2020

You might also use eehack's datalogger to see if the coolant temperature sensor is reporting erratically. That connector is on the front of the water pump and is susceptible to corrosion. An open circuit tells the PCM that the coolant is at -40F, which unlocks the converter and changes the spark and fuel quite a bit. An intermittent open circuit will cause frustrating hiccups but you'll only find it by plotting the coolant temperature over a decently long drive. I've seen that problem on multiple cars.

**Greezmonkey** · 05-02-2020

Thanks, during the shake down driving the last few months I have been using both EEHack and Scan 9495. Both have helped me figure out issues with external components. My package is pure stock with no component or engine mods. Thought it would be better to get the base system working good first. My temp readings have been very stable from cold start to running temp. With that said my fans are controlled separately via a Flex-i-lite variable temp unit. Fans come on at 195 at 60% then up to 100% speed at 200. Could that be an issue? Running down the road temps run on the thermostat(190) so I'm seeing 188-192. Could that be an issue?

**Greezmonkey** · 05-02-2020

Thanks Steveo, I do think it is about time to start getting into the ECM to make some necessary changes. This will be new territory for me so I'm Planning on going at it incrementally. With EEHack,TunerPro and a few good YouTube videos, I have been getting educated on the process. Looks like for starters I will need a good base LT1 Roadmaster bin file. Before changing anything I really want to make sure the base power train components are all working properly. Could leaky injectors causes a cruise speed stumble? They have not been cleaned or replaced. I also would like to know the basic ranges for the MAP,IAC and MAF for a stock set up. That would keep me from guessing whether or not they are reading properly. I've installed several aftermarket systems on vintage cars but haven't done ECM tuning going inside the unit. Thanks for the help!

**steveo** · 05-03-2020

Looks like for starters I will need a good base LT1 Roadmaster bin file.

grab flashhack or use eehack's built in flash tool and read the bin you already have, or find your calibration id in eehack and download your bin from here http://fbodytech.com/tutorials/bin-files/

Could leaky injectors causes a cruise speed stumble?

yes they could

They have not been cleaned or replaced.

they're ancient, clean or replace them before you continue and make sure they don't leak down (use a pressure gauge with engine off)

I also would like to know the basic ranges for the MAP,IAC and MAF for a stock set up. That would keep me from guessing whether or not they are reading properly.

the IAC should be at least 30 counts at hot idle, like to see 50+.

MAP should read vacuum. check against a mechanical vacuum gauge. barometric pressure should be your local barometric pressure

for the maf a good test for a stock car is if the car runs better with the maf unplugged, your maf might be no good (or you have a leak somewhere post-maf).. also usually a maf dies from having a fracture in its wire, so tapping it with a screwdriver may make the engine stumble, sure sign of a bad maf.

**Greezmonkey** · 05-15-2020

Hey Steve, just got the injectors cleaned and flowed to match OEM specs. Reinstalled and still have the stumble at cruise slow or fast. The lock up will go in or out too. Could you look at my data that I collected today during my drive to work? I have the morning drive and the return this afternoon. I'm just not completely sure what the data is telling me. The VSS data on EEhack is 25 MPH faster than actual. I have an Abbot speed corrector installed to improve the shift point because of the VSS difference. Not sure how this might effect the ECU.