Calling all 1.3 swappers-fuel mileage difference?

[ELVA164]

Hi all,

So last week, I decided to swap the 153 ECU from my '87 sedan for a 524 from an '88 vert we have here. I also tried a 173 from a parts car, but it seemed a tiny bit more hesitant (but that could have been just in my head). Both of the replacements, however, smoothed out the idle and cold running considerably.

I have noticed a pretty unfortunate side-effect to the swap: much worse fuel mileage. I used to be able to get high 20's and even low 30's pretty regularly if I tried to, but now the car struggles to achieve even around 25mpg. A drop of 5+mpg seems like a HUGE difference for the fuel mapping to account for, but is that something others have experienced?

[jlevie]

Assuming both DME's are good, there should be little to no affect on fuel mileage from a swap. But, the DME is adaptive and will adjust fuel trim to compensate for and engine running lean or rich. When you disconnect a DME (or the battery) that adaptation is reset and the DME will have to re-learn the necessary fuel trim. Also remember that these parts are 20'ish years old. While the digital portion of the DME doesn't degrade with age, the analog input and output interfaces can be affected by aging.

It may be that one or more of the sensors is not quite right or that there is a problem with the fuel system. For mixture control the relevant sensors are the AFM & it's IAT, the ECT, and the O2 sensor. Fuel system problems could be incorrect rail pressure or dirty/leaking injectors. Intake leaks can also degrade fuel economy and a smoke test will tell if any are present.

[dnguyen1963]

Drive around for awhile (including hwy). Your mpg should go back to normal assuming that you have not introduced any new problems during your swap.

[ELVA164]

Alright, thanks. I can't imagine that something in the fuel/intake system spontaneously broke at the same moment I unplugged the ECU, so I'll wait and see what happens and report back in a few days.

[jlevie]

Alright, thanks. I can't imagine that something in the fuel/intake system spontaneously broke at the same moment I unplugged the ECU, so I'll wait and see what happens and report back in a few days.

The more likely explanation would be that something was wrong all the time, but the 153 had adapted to the fault. When you changed the DME that adaptation was lost. While the new DME will eventually learn the system and compensate, the fault could get worse with time and lead to problems down the road. The better approach would be the fix/fix any problems that exist.

[ELVA164]

The more likely explanation would be that something was wrong all the time, but the 153 had adapted to the fault. When you changed the DME that adaptation was lost. While the new DME will eventually learn the system and compensate, the fault could get worse with time and lead to problems down the road. The better approach would be the fix/fix any problems that exist.

I'm sorry, I missed that bit in your first post. I definitely don't want to let a bad thing get worse, so I'll look into those items suggested the next chance I get.

As a side note, I took a short trip and drove as economically as was practical and got 28.0mpg on the OBC readout. Before, I could have gotten 31+mpg the same way. Just wanted to add some more solid numbers to it.

[ELVA164]

So here's an update. I've been using the 524 ECU since my last post. I've put at least a couple hundred miles on the car in all types of driving, and these are my findings:

1. Mileage is around 22-23mpg with mixed driving, down from 28-30.
2. Cold running and throttle smoothness are much improved.
3. Power seems to be down.

I've decided to switch back to the old ECU and see if things revert, then work on getting a smoke test done.