Fuel Economy Decrease

[Wilcrass]

I recently replaced the head gasket, water pump, timing belt & tensioner, cleaned the throttle body, changed oil and plugs.... My car is running really well. It got 50 mpg on the highway doing 55-60 over some hills. Then I got about 33 mpg coming back 200 miles, which was more uphill and going about 75 to 80. That seemed sort of normal but I had got closer to 61 or 62 mpg when driving 55 on some other trips. My mpg seems to have gone from about 43 mpg around town to about 28 mpg around town. Basically I feel I've lost some MPG although the car seems to be running fine. One thing that is different is the idle is a bit faster. I adjusted the distributor and that got it back to more where it had been. Is there anything I may want to try first to improve my fuel economy? I was thinking a slight adjustment on the distributor? It does seem to idle slightly faster. It's subtle though and the car is running great. It just seems like it's not getting as good of fuel economy. Thanks for any input!

[Jay2TheRescue]

Two things, you were going 80 MPH, and mostly uphill. This could be very plausible. Second thing, is your timing is probably screwed up. I don't know of any car whose official way to adjust the idle speed is to mess with the timing of the distributor.

[Wilcrass]

Thanks I agree that the distributor adjustment isn't the only factor. Adjusting it slightly does seem to affect the idle a lot. I reported everything I've been noticing to give an overall idea of the fuel economy. Perhaps the most striking observation is that my around town driving seems to have gone from an average of 43 mpg to about 28. I was thinking that it has to do with cleaning the throttle body somehow? Or that perhaps the vtec-e is kicking in differently. I'm curious if cleaning the throttle body could have affected the o2 sensors? We didn't really do much to the head. We just resurfaced it slightly, the least amount to clean it up. What's misleading is that the car is running well and even seems to idle well, it's just not getting the exceptional mpg that it was. One peculiar trait was the idle going up and down when rolling to a stop. Adjusting the distributor greatly reduced this, making it almost imperceptible. I start to question how I'm driving and wonder if I'm not being consistent. But the numbers seem so different that it does make me think that there is some tuning issue. So what all should I try?

[Jay2TheRescue]

Idle varying... Look for a vacuum leak, possibly a vacuum hose that may not have been hooked up when everything was put back together.

[Wilcrass]

Thanks, we did check for a vacuum leak and didn't find one. I adjusted the distributor the slightest bit more and it seems to have settled into idling and performing how it had been. Now I'm going to be especially careful to drive conservatively and see what my MPG's are! I appreciate your input!

[theholycow]

What do you drive? I'm going to guess a Civic VX.

You replaced the timing belt and tensioner. Could the old one have been stretched and affected the timing?

You replaced the water pump. Was the old one not working as well as it should, and the engine was running warmer?

Could external variables have changed? For example, weather or fuel supplier?

[Wilcrass]

I do drive a civic VX and the old timing belt didn't seem too bad. The water pump had a very slow leak out of the weep hole which suddenly got much worse. There was a puddle of coolant under the water pump and I decided to replace it. We went ahead and replaced the timing belt and tensioner. The old one wasn't too bad. Another issue was that one of the plugs was cross threaded and broke off, leaving the bottom half of the plug in the head. So we took the head off to get the plug out. It came out fine and the threads were good. We had the least bit of the head resurfaced and put a new head gasket. It is running great except for the possible MPG loss. I realized I may not have been as diligent with my hypermiling (coasting) and may have been driving just slightly less conservatively. I'm going to be especially mindful for a bit and see if the MPG is down as much as I was thinking. I'll report back in a few days. Weather and fuel are mostly the same. Thanks for your input! I appreciate it.

[ukrkoz]

1. First thing you do after such a job is to pull ECM fuse out for overnight. Some also say to remove battery cables and touch them. To remove any residual ECM charge. What this does is it resets all previously stored driving habits your ECM had.
2. I am big Seafoma fan. Run 2 cans through 2 tankfulls of gas. It never hurts and always helps.

[theholycow]

Does 1992 technology store driving habits? I thought that sort of thing didn't become common until another 10 years.

[Jay2TheRescue]

I don't think it stores driving history, but I think it does store previous data from the O2 sensors so it can update the fuel maps.