Very poor fuel economy, idle adjustment problems.

[Vakito]

Hi all, I've had my 1988 240 for a year now and all that time I never gave a thought about how much fuel I was actually using. Recently I did the maths and worked out I'm only getting around 12-15MPG!! It's a 2.0L B19E with K-Jet, no ECU, AW70 non lockup gearbox, brakes are fine tyres are fine engine runs fine and I don't drive around like Sterling Moss.

What I've done so far to address the problem is had the injectors ultrasonically cleaned and refitted with new seals for the injectors and the holders. New fuel filter, new spark plugs and new air filter.

The air filter box's hinges are broken at the bottom causing it to not close and seal properly and I'll need a new one shortly. While I was changing the filter I thought i'd remove the flap valve to test it and it's stuck closed so it will need repairing.

After replacing the injectors I started the car up but it would not idle without stalling as soon as I took my foot off the accelerator, this was easily fixed by adjusting the idle speed screw on the throttle body. I'm not sure if this is a good sign that the engine now needs more air for the same amount of fuel or is it something else?

Also regarding the idle adjustment screw, the adjustment doesn't seem to be linear, you could screw it in or out for a while with no change and suddenly the idle speed will adjust erratically.

I've not taken it out on a run since I've replaced these parts, just started it up on the drive but what I want to know is what's the deal with the idle adjustment screw and is there anything else I can do to improve fuel economy?

Peter

[Bob Meadows]

Peter: ~ the throttle body internals may benefit from a clean-up—carburetor cleaner should help, I have a spare body stripped –down just in case! The idle screw should give an almost instant change to the engine note; the same can be screwed right out for cleaning.
It has an “O” ring attached to the spigot so also worth checking (may need to be pulled out once the threads are clear of the body) Redex added to each petrol fill will also help to keep the internals clean to include the jets.
The air box can be repaired at the base with a small length of aluminium angle section to replace the hinge clamps that hold the front in place (had the same problem) the flap valve system is better screwed down so it remains closed i.e. eliminate the small thermostat valve actions (see various advice/recommendations on this) the snorkel can also be removed and replaced with a grill end allowing better breathing for the engine
Hope it helps
Bob

[Vakito]

Thanks for the advice, I actually did clean the throttle body around 5000 miles ago but I guess it wouldn't hurt to clean it again, I'll get on with it tomorrow.

Regarding the o-ring on the idle screw, do you know where to get replacements?

[Bob Meadows]

I doubt if the ring is anything special in terms of size etc.:~ for these I normally go to my local hydraulics supplier or you could try engineering supplies for a quality item.
Regards Bob

[Vakito]

I'll get it measured tomorrow and try find a replacement. Cleaned the throttle body again today but it was still very clean from last time.

There is a much larger drop in RPMs when selecting drive or reverse than I had before (around 500RPMs now compared with 100-200 before) and the idle speed is a lot slower when the engine is cold, if I adjust the screw to have the engine idling at 850 when cold, it will idle at around 1300 when warmed up.

[dingov70]

Having read your post about poor repsonse to the idle screw & sudden leap in revs , I would pretty much bet that the optentiometer in the throttle body is goosed .

many are carbon track based & the carbon wears out or breaks leaving gaps that the ECU struggles to work out whats required .

You could try using a mulitmeter to read the resistance changes as you operate the throttle body , they SHOULD be a consistant rise / fall in value . IF you get sudden changes , there is your problem .

No I do not know which pins to read from , but I am sure another merry soul does know that bit

Good luck

[Vakito]

Hi, thanks for the advice but sadly mine is completely mechanical, no ECU no sensors.

Peter

[Clan]

Originally Posted by Vakito

Hi, thanks for the advice but sadly mine is completely mechanical, no ECU no sensors.

Peter

what is the CO gas reading ? should be set to 1.5% ….

[Vakito]

Sorry still very unfamiliar with kjet, how do I test it?

[Clan]

Originally Posted by Vakito

Sorry still very unfamiliar with kjet, how do I test it?

Only a garage will have a exhaust gas analyser , mot stations will have one , .. there is a hole about 5mm diameter between the fuel distributor and air flow plate on top the alloy housing , you need a long 3mm allen key to adjust CO , but do not do trial and error as you will loose your base point where it will run , the adjustment is very sensitive .,