I am getting dreadful mpg ?
Discussion
I've noticed that over the last few weeks I seem to be using a lot more fuel than previously. So I've just checked my mpg (brim full and then run the car and then brim full at same pump). So I checked everything and quick calcs suggested about 15 mpg.
So, I gave the car a good service, new plugs, leads, cap, rotor, air filter, oil filter, oil change and fitted new ECU temp sender. The car runs well, starts well, tyre pressures checked and all as usual (24 psi), uses no water, runs at right temperature (I use the LR temp sender+resistor for dash temp gauge).
I deliberately not driven the car hard whilst I checked the mpg more closely.
Result is I am getting average 16 mpg on a good single run of 150 miles and around town just average 9 mpg on 80 miles over 6 runs out.
In the past I was getting 270 miles to a tank full, now its down to 150.
Any ideas what could be at fault ?
So, I gave the car a good service, new plugs, leads, cap, rotor, air filter, oil filter, oil change and fitted new ECU temp sender. The car runs well, starts well, tyre pressures checked and all as usual (24 psi), uses no water, runs at right temperature (I use the LR temp sender+resistor for dash temp gauge).
I deliberately not driven the car hard whilst I checked the mpg more closely.
Result is I am getting average 16 mpg on a good single run of 150 miles and around town just average 9 mpg on 80 miles over 6 runs out.
In the past I was getting 270 miles to a tank full, now its down to 150.
Any ideas what could be at fault ?
TimJM said:
I had a problem once with the AFM which resulted in very low mpg. This may be different to you though as the running was lumpy too and noticeable when driving. A new AFM and back to its usual 20-25mpg self.
I've just taken a look at its fitted with a 3AM, odd for a '98 400. (Curious but it still had that when it was giving good mpg......)I wonder how can you check a AFM is operating properly ?
Have you reset the ECU, or checked the Lambdas?
IMO the best easy check is to have a session with an ECUMate www.ecumate.com
IMO the best easy check is to have a session with an ECUMate www.ecumate.com
V8 GRF said:
Have you reset the ECU, or checked the Lambdas?
IMO the best easy check is to have a session with an ECUMate www.ecumate.com
Cheers V* GRF, more good points....... IMO the best easy check is to have a session with an ECUMate www.ecumate.com
I have now reset the ECU, I have a battery isolator fitted so that's easy.
Haven't checked the lambda's, don't know how so will now toddle off and see if I can find how.
Rather than ECUMate would the use of Rover Gauge be an alternative (providing I get the right Rover V8 14CUX interface cable) ?
Testing the AFM....
Found this site useful http://www.rv8r.co.uk/standardecu1.html
Testing AFM (Extract)
1. Insert the negative probe into the Red/Black wire (sensor ground), and the positive into the Blue/Green wire (Airflow signal).
2. Turn on the ignition, but do not start the engine.
3. The meter should immediately indicate a reading of approximately 0.3-0.34 Volts. Most defective airflow meters will overshoot to 0.8 Volts or higher, and take at least 2 seconds to come down to the correct voltage.
4. Now start the engine, and the reading should rise to 1.6 Volts (3.5 Litre engine) to 1.75 Volts (5.0 Litre engine).
At 3 I got 0.38 (expecting 0.3-0.34
At 4 I got 1.68 (expecting 1.6-1.75
Found this site useful http://www.rv8r.co.uk/standardecu1.html
Testing AFM (Extract)
1. Insert the negative probe into the Red/Black wire (sensor ground), and the positive into the Blue/Green wire (Airflow signal).
2. Turn on the ignition, but do not start the engine.
3. The meter should immediately indicate a reading of approximately 0.3-0.34 Volts. Most defective airflow meters will overshoot to 0.8 Volts or higher, and take at least 2 seconds to come down to the correct voltage.
4. Now start the engine, and the reading should rise to 1.6 Volts (3.5 Litre engine) to 1.75 Volts (5.0 Litre engine).
At 3 I got 0.38 (expecting 0.3-0.34
At 4 I got 1.68 (expecting 1.6-1.75
Cheers Peter (Jojackson)
I don't think there's any doubt its running rich.
Over riding factor is that I now get only ~150 miles per tank vs ~270 in the past and other symptoms point to running rich too, sooty exhausts, smell of unburnt fuel.
I don't think there's any doubt its running rich.
Over riding factor is that I now get only ~150 miles per tank vs ~270 in the past and other symptoms point to running rich too, sooty exhausts, smell of unburnt fuel.
Edited by blaze_away on Friday 27th September 19:14
blaze_away said:
Cheers Peter (Jojackson)
I don't think there's any doubt its running rich.
Over riding factor is that I now get only ~150 miles per tank vs ~270 in the past and other symptoms point to running rich too, sooty exhausts, smell of unburnt fuel.
150 miles per tank is too low. When I had the AFM problem I referred to earlier 1/3 of a tank only managed 30 miles so I would have been about 90 miles on a tank. I don't think there's any doubt its running rich.
Over riding factor is that I now get only ~150 miles per tank vs ~270 in the past and other symptoms point to running rich too, sooty exhausts, smell of unburnt fuel.
Here I am. 
You can guess as much as you like, but diagnostics are the answer. The problem is the the TVR ECU warning light may not even be wired in, or the check light facility enabled in the ECU chip, so you dont know if the ECU has an issue. Assuming your car has catalysts, the ECU should tightly control the mixture, but will fail to do so if something has failed, typically an AFM or Lambda probe. Both ECUmate or RoverGauge will give you much the same information in terms of Lambda (RoverGauge gives extra long term trim data ) and AFM operation, plus error codes, its up to you, if you dont mind using a laptop with a £35 cable or an £120 ECUmate as a stand alone tool. You can check both Lambda and AFM operation with just a test meter, but you wont get error codes from the ECU.
Cables are on the PH classified,
http://www.pistonheads.com/classifieds/parts-and-p...
Test meter tests are on www.g33.co.uk under technical- fuel injection.
You can guess as much as you like, but diagnostics are the answer. The problem is the the TVR ECU warning light may not even be wired in, or the check light facility enabled in the ECU chip, so you dont know if the ECU has an issue. Assuming your car has catalysts, the ECU should tightly control the mixture, but will fail to do so if something has failed, typically an AFM or Lambda probe. Both ECUmate or RoverGauge will give you much the same information in terms of Lambda (RoverGauge gives extra long term trim data ) and AFM operation, plus error codes, its up to you, if you dont mind using a laptop with a £35 cable or an £120 ECUmate as a stand alone tool. You can check both Lambda and AFM operation with just a test meter, but you wont get error codes from the ECU. Cables are on the PH classified,
http://www.pistonheads.com/classifieds/parts-and-p...
Test meter tests are on www.g33.co.uk under technical- fuel injection.
Edited by blitzracing on Friday 27th September 21:15
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