Mileage on a full tank of petrol has dropped by at least 200km

Thanks for the advice. This is the most sensible approach, at least in terms of eliminating any faulty parts in the engine through the codes check. I'll keep everyone posted on what the mechanic says when he tries all the things you have suggested.

Well, here is the situation so far:

I had the original people who did the repairs on the car just before this fuel consumption problem began to check this problem. Apart from trying to avoid the issue a few times by repairing a few other things and the guy I spoke to seem to suddenly have something in his throat to clear when I mentioned the problem, it would appear from their testing of the codes and checking fuel and oxygen ratios that they thought everything is okay. I mentioned about spark plugs and I think they checked it, but they couldn't find a problem.

So I took the car to a proper Honda dealership with its own mechanics. They found the spark plugs were in a shocking state and needed replacing (which they did). They also replaced the fuel filter claiming that a partial blockage would see the computer controller telling it to push more fuel through. Seems reasonable. They also decided to reset the computer controller saying it does an initial learning of my driving habits and adjusts things like the amount of fuel going into the engine. Well, after these repairs and running the car for more than a week, I can be certain the problem is not fixed.

So now I am thinking the computer controller itself is what's causing the problem. Either I have an older model controller from an earlier Honda CRX Del Sol, or there is something wrong with the controller. However, a check of the codes is not showing a problem. So maybe I have a swapped computer controller?

Is it possible the computer controller is the cause of this sudden increase in fuel consumption from 600 plus km down to 400km? Or could someone have fiddled around with the odometer (for what reason I would not have a clue).

Can anyone figure out what other possibilities could be causing this increase in fuel consumption?

I have further details from mechanics about this sudden increase fuel consumption problem on the Honda CRX Del Sol. See below for my comment.

Any further thoughts?

The mechanics can confirm it is not a weight of the car issue (even though a heavy car would require more fuel to move it). My car has not had a change in weight before and after the increase fuel consumption problem kicked in. Definitely no heavy bricks in the boot to carry around. About as lightweight as one can get a Honda CRX these days.

And no, I definitely have not attached a brick wall to the front of the car to increase air resistance to the car (which would increase fuel consumption).

Fair enough.

Funny you should mention vacuum leaks. I mentioned your advice about checking for vacuum leaks with the original mechanics and they claimed not to have found any issues here. But then again, I mentioned the spark plugs too and they couldn't find a problem either until a proper Honda mechanic noticed how bad they were.

Latest update is that after the change in spark plugs, new fuel filter and a reset of the computer controller, the orange light indicating it is time to re-fuel comes on with odometer now reading at 310km. It seems the repairs have made it worse.

Well, if the odometer is rigged, perhaps the original mechanics are quietly hoping (given the vgc of the engine and car itself) that I sell it to them because of this "alleged" fuel consumption problem? Yeah, wishful thinking!

I have noticed some guy talking about ". SEA FOAM and fuel injector cleaners" and after doing the fuel injector cleaning, the Del Sol traveled a lot further for him. He also mentioned to check tire/tyre pressure and wheel alignment claiming it makes a difference. Certainly I am due for new 4 x tires/tyres.

I will mention the vacuum leaks and cleaning fuel injectors to the Honda mechanic, and I'll get some new tires/tyres.

Beyond that, I am going to check the actual distances I travel to work and back using a GPS system to see how many km I do and multiply that for the number of times I travel to work and work out the total km. Then maybe I will know if the odometer is playing up.

That's for the advice.

Any further interesting developments and I'll mention them here for the benefit of other Del Sol owners.

Thanks.

Checking the odometer on the Honda CRX Del Sol

I used a Subaru 4WD Forester for this test. It comes with a digital odometer, which may suggest reasonable accuracy.

Odometer readings
Start: 177,411 (from home, on Wednesday morning 23 September 2015)
End: 177,480 (arrived at work roughly 45 minutes later).

Route taken
Shortest and quickest route along the only highway into the ACT, then take a right along Gunghalin Drive to Woden, and a final twiddle along some street roads to the work location.

Distance travelled
177,480 km †177,411 km = 69 km

A one-day trip to and from work along the shortest route is 69 km x 2 = 138 km.

Comparison with Honda CRX Del Sol
The number of times to work and back home on the Honda CRX Del Sol immediately after the last repairs (to replace spark plugs and fuel filter) was 4 round trips, or 8 trips of 69km assuming shortest distance. This was for the period of Monday afternoon of 14 September 2015 (when I picked up the car from the Honda mechanic), the 3 days that followed, and on Friday morning of 18 September 2015. Total distance: 8 x 69km = 552km. Yet the Honda CRX had indicated less than 350km on the odometer.

In reality, I don’t always take the shortest route. On some days I will visit Canberra City, Dickson Shops, Kippax shops and Gunghalin shops. If this was done on the Subaru (assuming it can cover the distance on a single tank of petrol) or Honda CRX, it would almost certainly reach 600km by the end of the fuel supply.

The Subaru will certainly not reach 600km on a single tank of petrol. The Honda CRX Del Sol, noted for its economical use of the petrol, can reach this distance and even go further, up to 650km on premium 98 petrol, good tire pressure, and good driving behaviour.

NOTE: The previous 1987 CRX model could achieve 600km as well.

This is strongly suggesting the odometer is either faultering or has been tampered with.

Given how quickly the change the odometer readings were before and after the first repair with the other mechanics for replacing the starter coil (the initial repair), I suspect that there has been some tampering of the odometer.

I guess the next step at this stage is tol ask the Honda mechanic to see if he can check the status of the odometer and whether there is anything odd about it.

Okay. I believe the cause of the problem is almost certainly the pulse analogue sensor that measures distance travelled. The Honda mechanic found no problem with the speedometer and odometer (and no signs of tampering). Therefore, I am left with the sensor itself as the part to cause the problem.

Something has happened to this sensor. Has this sensor been replaced by an older or faulty sensor? Most probably. Can I prove it? Probably not. All I can do is see if I can get a new replacement of the sensor.

Thanks to everyone who provided advice.