Engine hunts when hot

[LenChim]

OK 4.5 Chim 2001. Engine starts ok, seems to run ok, if fact like the proverbial of the polished shovel but when it gets hot the tick over goes 2000 to 100 rpm approx and makes it almost undriveable. It is also started drinking fuel as though it is going out of fashion! 15MPG up to this poin it has been 22 mpg. I have looked at the air control servo motor, it is clean and moves in and out ok when driven by the Rover gauge from the computer. I have had this the car a few years and have replace the air control servo motor on this car about a year ago. I don't want to go down the rout of changing ecu's upgrading this and that, I just want to keep it as std as possible. Has any on ideas?

[LenChim]

Thanks, I came to the same conclusion, I only know one guy with a TVR localy but the weather has conspired against me so far but will get it tired out in the next few days.

[LenChim]

If you have one knocking around that would be great. Many thanks for looking.

[LenChim]

Once when I unplugged it the hunting stopped but not the next time. So possibly it was in proximity the right places and way too far out the next?

[LenChim]

Ok I will look at the afm.

[LenChim]

blitzracing said:

Try this from Mark Adams- the critical bit is the warm up spike:

Most airflow meter faults will cause the engine to run excessively rich. However if the airflow meter remains connected whilst defective then the vehicle will probably not run. In most cases the output from a defective airflow meter will be in the range 2.0-2.5 Volts, which is a viable value. This represents a moderate load and will cause heavy over-fuelling without setting a fault code.

Testing is performed in the following manner. Peel back the rubber boot on the airflow meter connector and leave it plugged in to the airflow meter. Set up the digital multimeter to read voltage. Insert the negative probe into the Red/Black wire (sensor ground), and the positive into the Blue/Green wire (Airflow signal).

Turn on the ignition, but do not start the engine. The meter should immediately indicate a reading of approximately 0.3-0.34 Volts after the initial "warm up" spike. 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.
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).

Ok , Now I have measured the voltage ignition ON 1 volt, with the engine running I get 1.9 - 2.0 volts. I have an analog VM. It does have a 0.1 volt scale so I would have thought it would be reasonable accurate on a 3 volt scale but I have just asked a friend if he has a more accurate and calibrated unit to try. So just waiting to see if he can come up with the goods.

[LenChim]

Ok I see the line ins that from an oscilloscope? Any way whilst I am waiting for a better piece of measuring kit i have been looking on line for afm's it seems there are at lot out there, TVR parts tell me that the afm is the same on a Chim 4li through to 5li, if that's the case is it the same as a Land Rover V8 unit an ERR5198G? £55.00. TVR part number E2036 @£189.55 +VAT =£227.46 But and there is always a but this item looks to have the same plug socket but it has 6 pins in it my unit on the car has only 5 ins but has a space for the sixth. is there some other function that I do not know about in there?
This ERR5198G is supposed to be compatible with the 3.9 li 211HP Rover all the way up to the 6li Jag 311HP.
Any one had any experience with this?
Many thank.

[LenChim]

OK a bit more information from my car, first off I thought I would have a look in the AFM so I took the castings off each end and had a look down the hole where the wire and resister are, it all looked a bit dusty so I gently blew it out, then I injected some paint thinners through it and blew it out again, after several cycles of this I could then see that the wire looked like a coiled spring not a solid lump. Reassemble and put it back om the car.
Voltage test red/black -ve blue/green +ve ign, on, 0.352v. Engine running 1.98v. Engine hot 1.74v.
I took some pictures of the screen running the Rover Gauge. Cold, Hot and at 3000 rpm.The last one is of the state of the floor after running, this I suspect is due to the temperature and the fact that the car has been started many times now without going for a run to clear it out.

[LenChim]

[LenChim]

[LenChim]

blitzracing said:

Couple of things -Your short term trim is hitting 100% on one side, which is wrong, it should be cycling around the mid point. This means either the long term trim has not been set up as the battery or ECU has been disconnected recently (?), or a sensor is a mile out and the long term trim is at 100% and run out of range. To reset the long term trim (if its near 0 at present) you have to let the car idle for at least 2.5 mins when hot (above about 90'c) and you should see it slowly move as the ECU learns the new setting. Once its stopped moving, the short term trim should then cycle around the mid point. The lower the long term trim, the better the map and sensor data is for your engine, and on a TVR 30%or below would be a good value. As for the AFM reading- set it to direct, it should read between 32 and 35 % at idle- Id expect a 4.5 to be midway between these 2.

Edited by blitzracing on Thursday 12th December 16:41

Ok, before I started the car today I looked at the fault codes and cleared them. As I did not clear them before the last run not sure that they are relevant.



I also looked at the "Battery backed RAM Contents" Not sure what they mean or if any are incorrect.



Ran the engine to 90C idled for 3 min. MAF @ Direct, Lam Trim LT.

[LenChim]

Then I switched to short term Lam, it was very variable.
ODD EVEN
-28 -13
-24 -20
-39 -16
-23 -22
-40 -19


Then I took a pic of the LT Lam


I also looked at the "Fault code file" again and it said the the "Air control stepper motor was at fault" But that was probably due to the fact that I pinched off the air pipe to it to reset the tick over with out the ACSM coming into play so it was having a sulk.

[LenChim]

Ok the latest screen shots;.

Right I have got the car hot 90c ish let it idle for 3 min. the screen shots show the results Short term trim is about mid point but the log term is -85/-57, the AFM seem to be ok at 33%.
The car seems to run ok, no shunting but runs out of revs at 5000rpm, seems to ether run out of fuel or sparks cant really tell which, I replaced the HT leads about 3 months ago.
I have to say that getting the car up to temp is a problem when running around, I changed the stat to a 82C unit some time ago may be I need to go to 88C. The Ignition timing is set at 8 deg BTDC with vac off at 1000 rpm.I also need to change the temperature gauge sender as only reads 70C at about 90C eng temp. Only gets to 90C standing in the garage.

[LenChim]

[LenChim]

blitzracing said:

Even though RoverGauge only reads trim to 3400 rpm, you can still pop a meter on the lambda output to get a passenger to read the voltage directly. It should cycle between 0 and 1.2 (or so) up to 3400, then stick at around 1.2 volts or slightly over to peak RPM. If you are running out of fuel it will drop to 0 volts. Problem is a bad misfire also looks like a lean mixture, so the results may not be that different. If you want to diagnose HT faults, and old style Crypton tuner is the way to go where you can monitor the HT wave forms. Shorts show as low HT, and open circuits show as higher voltage spikes. Mind you it may cost you as much to diagnose as swap all the suspect bits out. Its a pity Gunson used to do a brilliant HT tester with a simple LED bar display for the HT voltages, that tells you a lot, but have long since been out of production.

OK Christmas is over so I have made the plug assembly's to go in line to check the lambda out put and ran the test today. The outputs were good once the engine had warmed up. On the road the at constant speed they seemed to cycle ok, decelerating and accelerating but at about 3300RPM they switched off zero volts.
So the question is were do I go from hear? I don't have a Crypton tuner, as I said I have replaced the HT leads but they are tied together. I did take the cap of the dizzi and clean it and look for cracks, tracking lines etc. It does not sound as though it is misfiring. I have rechecked the fuel pressure its ok, also when revved hard but I can not test it under load (no rolling road).
Any suggestions.

[LenChim]

[quote=blitzracing]

The lambda switching you describe is correct- it drops to zero on the over run as there is no fuel going in- BUT it should not drop to zero as the RPM increases the voltage should rise to around 1.2 volts at 3400 and stay there to peak RPM. If it does drop to zero, the engine is running horribly lean or misfiring really badly. Lean running can be a blocked fuel filter, poor fuel pump supply voltage (or connections)or crushed fuel line. Checking fuel pressure is tricky, but can be done by removing the valve from the pressure point take off,(serp engine only) and attaching a fuel line to an oil pressure gauge that you can lodge under a wiper blade so you can read it inside the car under load. Fuel pressure should be 2.5 bar at full load and RPM.

One question, it may be stupid but could the lambda sensors be wired back to front?

[LenChim]

blitzracing said:

Not easily- it is possible to short the heater wire to lambda output, this puts 12v on the ECU input, that will make the ECU think the mixture is rich, but in this case both the short term and long term trim is stuck at 100% removing fuel in RoverGauge. If the short term trim is moving, the probes are working. One thing I will say is loss of fuel pressure or lean running under load wont cause hunting RPM or a misfire at low RPM when the fuel requirement is low, I don't want you to chase faults that may not be there, as the results are so inconsistent. If you think you have a misfire, just try melting a cable tie onto each exhaust header on a warm engine, as its less likely to melt on a cylinder thats not firing correctly.

Right next installment. I have checked the fuel pressure when driving it is fine drops a little on over run but is 36 psi on acceleration.
The Voltage to the lambda sensors drops to zero after 3300rpm app.

As can be seen at,
rpm 5248.
Lambda STD "0"
MAF 78%
Throttle 59%
Temp 80C
Fuel pressure 36PSI
The engine feels like I have hit the rev limiter.
By the way this was in 3rd gear. Or I would be totally illegal.
Info from other screens.
RG RPM is very similar the the cars RPM.
RG 5248 rpm (misfire or running out of power) rpm 4752 matrix segment DE.
RG MPH reading always seems to max out at 40MPH and starts about 30 as soon as you are moving.

[LenChim]

blitzracing said:

The lambda readings in Rover Gauge are fuel trim values, not the voltage that comes out of the probe. It will drop to zero in RoverGauge as the ECU stops monitoring the lambda probes at 3400 rpm as its not longer under emission control. What we need is the actual voltage from the probe with a test meter, as this will still be there even when RoverGauge is ignoring them- so this is where you need 1.2 volts or so under full load with a test meter. Fuel pressure tests really need to be done at full load, peak RPM, not just the acceleration phase as the fuel demand wont be at peak until l 5400 rpm but that's likely to be at around 100 mph in third gear from memory in my car (TVR may have different gearing), so its not that easy to do without getting noticed - Mind you something as simple as a failing HT coil will cause a nasty misfire as the RPM rises and the spark potential demand is at its greatest, and the dwell period at its shortest The readings you have there seem OK, but its only what the ECU sensors see and the ECU is not that smart .

Edited by blitzracing on Sunday 12th January 17:17

I tested the voltage on both the lambda's with a voltage meter set at 3 volts fsd, both cycled well until 3300 rpm approx then they both dropped to zero volts, me driving and passenger watching the volt meters and RG. He was also watching the 40 psi pressure gauge plumed in the the injector manifold, and yes we were under full load two up and 90 plus mph when when we ran out of rev's at about 5400 rpm. They was the results that I posted earlier.
I have a couple of spare HT coils so I will switch out the current one and road test when the weather is a bit better.
Many thanks guys for sticking with it.

[LenChim]

TV8 said:

Hi Len, I had a couple of people contact me about the AfM but checking you have ruled that out as ok? I had running problems and that was temperature dependant and caused by the AfM. Can you hear the stepper motor resetting when you switch the car off?

Yes I can hear the stepper motor reset when I switch off.

[LenChim]

blitzracing said:

So just checking you still have full fuel pressure on the injector rail at WOT (it will drop at less than full throttle) and you still have the lambda output dropping to zero? That would be a really odd combination, (assuming it not ignition) as basically the map data would have to be incorrect as you have full pressure behind the injectors, and the map controls the injector timing. What brought this fault on? Its not a second hand ECU or something silly from a Range Rover? Double checking what map is it? Its normally map 5 for cats.

Just took the coil off and measured the resistances they are as the book, of course that does not mean it is not braking down under load. The ECU has been in the car since I got it and I have not noticed this problem in the past but as I have had all the other problem crop up maybe it was there all along so I have been testing the car's capabilities and now have found it. As to the map how do I check that?