i am running the ARAP bin on my 89 Iroc, with the 165 ecm, I got into chip burning hoping to increase fuel mileage, because i was running way rich (108 BLM in all cells) I have my injectors up to 24.5 and still recieve 108-116 readings.

Heres the question, while driving and scanning with craig moates software, i noticed that my vehicle speed would jump around on avg 20miles, so say i was doing 40mph, the vehicle speed would say 25 one second then on the next update it would say 55-60, could this be what is causing my increased fuel consumption? do i just need to change the vehicle speed sensor.

 

change the vss also make sure the wiring is sound. it very well could be dropping in and out of highway spark mode.

 

ya i forgot to mention that, i set all the settings for highway mode, and highway spark, it only gets into highway mode for 1-2 scan updates, then drops back out, i know because of watching the A/F ratio go to 15.32 from 14.71

 

And don't forget the TCC is engaged/disengaged by the ECM based on VSS input, if it is constantly engaging/disengaging you will get crappy gas mileage and poor performance.

 

Yes, absolutely. A non-functioning VSS can result in a solid 1 - 3 mpg loss on an automatic car. I could plainly see it when I had a VSS problem, as I chart my fuel consumption in Excel. As soon as I fixed it, my mpg went up 3 mpg on average.

 

ok, this may sound retarded, but where is the VSS located at? i have read that it is located on the back of the spedo, is this correct?

 

I'm not sure on the 89's but on older ones, yes it is on the back of the instrument cluster, held in place by 2 screws, all of the ones I've seen were a yellow box about 1 inch wide and 3 inches long and half an inch thick

 

yup, thats what mine looks like. Where can i buy one, i called napa and the list an ignition speed sensor, its like $75 the guy said he didnt think it was what i was looking for, since the vss gets its info from the revolution of the speedomoter, could my problem be caused by the fact that my speedo bounces all the time?

 

The sensor has a photo electric let that points at hte speedo head and gets its speed readings that way...ignition speed sensor? Never heard of it.

If your speedo is bouncing around a lot it could be causing the TCC to unlock/lock a lot, but you'd feel that in a jerking of the car when driving near the speeds where the TCC actuates.

 

there isnt a jerking of the car, so i dont think that its the torque converter, i was led to believe that it was the cable needing lube, but would the bounce cas the irregularities in the vss reading?

 

here i have attached a excel workbook that compares my speed readings to my RPM, if you guys could take a look, i am sure you will see what im talking about, and maybe be able to help

 

Yes, the bounce would cause VSS irregularities because the cable is driving the VSS mechanism.

The first thing I would check is the mechanical connections (ie. the cable to gear housing, the cable itself, and the cable to cluster connection). 89s have a two-piece speedometer cable. You can lube the cable without pulling it out of the car, but you'll have to pull the gauge cluster.

Then I'd make sure the electrical connections are clean and making good contact. The two points to check are both on the laminated circuit on the back of the gauge cluster. Since you'll have it out anyway you can do it at the same time. You can clean it (carefully and rubbing in one direction not sideways!) with a rubber eraser.

I'd also make sure the optical pickup is dust-free, and that the spinning metal bar (which chops up the light pattern and creates the speed signal) is clean and reflective/rust-free.



Did you say your actual speedometer needle bounces, or were you just talking about the scanner reading?

 

yes, the needle actually bounces, not too bad about a 10mph range, and its a little off, cus i went from stock 15's to 16's with a 225/60/16 tire one, the reason for the larger tire is because the tires were free from work (tire store)