4.3BV on the rollers - faulty map tune resistor

[FactBV]

I took my recently acquired 4.3BV down to Surrey Rolling Road today to see what sort of power it was making before I start messing with it. It pulled cleanly to 239 bhp at 4700 rpm and then started miss firing badly. The Air Fuel Ratio (AFR) at the point where it starts to misfire is 15.8 - 1 (see attached printout hopefully) I used Steve's ECU mate to check for fault codes when I got home and it records a faulty map tune resistor. The one fitted to the car is a green one, which from the ACT website should be 470 ohms. I checked mine and it is showing 471 ohms so no problem there.

The AFR graph is just about a 45 degree line from 10.5 - 1 at 1500 rpm to 15.8 - 1 at 4700 rpm which doesn't look right. Would the lean AFR cause the misfire? Has anyone had experience of this, any ideas?

The problem is that during the last couple of weeks I have changed all the breather pipes, HT leads, rotor arm, coil, HT cap etc. I have changed things in stages and taken it for a run in between so that I can check what I have fitted but this fault only really shows under load.

Thanks in advance.

[HarryW]

The fact that the AFR seems to ramp linearly with revs says to me it's not climbing the map with the increased load, which would lead me to look at the AFM output. If its a 5AM AFM it should be peaking at 5V. I'm surprised Charlie took it so far up the rev range with such a weak AFR, IMHO you could be doing damage.

[FactBV]

HarryW said:

The fact that the AFR seems to ramp linearly with revs says to me it's not climbing the map with the increased load, which would lead me to look at the AFM output. If its a 5AM AFM it should be peaking at 5V. I'm surprised Charlie took it so far up the rev range with such a weak AFR, IMHO you could be doing damage.

Thanks that's really helpful.

So would an AFR of 15.8 in its self (when it should be 14.7) regardless of what is causing it, result in the misfire underload?

Also, the fact that I have the tune resistor fault and the AFR climbing linearly with revs; would that make an ignition fault, for example, unlikely to be part of the problem?

Having changed so many parts recently, it will almost be a relief if it is the AFM or a map issue which means that I don't have to go back and start refitting the old parts to work out which part maybe a problem.

I am using the ECU mate for the first time but note that I can use it to check the AFM hi and lo voltages while the engine is running, they should rise together and as you say peak at 5 volts, so I will try that tomorrow.

Thanks again for your help.

[HarryW]

B-Reight said:

Is it just me or does the 45 degree line show the horsepower rather than the afr? Seems to correlate with the 239hp peak.

Also, full throttle afr should be in the region of 13.2 to 12.6.

Edited by B-Reight on Saturday 8th June 23:43

Looking at it again, you are right, I thought it strange Charlie would continue with such a strange AFR response.......

[FactBV]

B-Reight said:

Is it just me or does the 45 degree line show the horsepower rather than the afr? Seems to correlate with the 239hp peak.

Also, full throttle afr should be in the region of 13.2 to 12.6 throughout the rev range.

Edited by B-Reight on Saturday 8th June 23:32

Ahh you could well be right. The guy said that it was was running lean and pointed to what I thought was the straight line but the profile of the straight line is identical to the bhp on the first page.

So, given that the AFR is 14 (not between 13.2 and 12.6), before it started misfiring, I guess that would not be enough to cause the problem, or would it? I will check the AFM voltage tomorrow anyway and see what it looks like. Many thanks.

[Hoover.]

Pete,

Even though you were looking at the wrong line on the graph the comments above still stand, as the AFM should be a horizontal line......

Had massive issues with mine, as it doesn't seem to want lay flat and overfuels a hell of a lot.

[FactBV]

Hoover. said:

Pete,

Even though you were looking at the wrong line on the graph the comments above still stand, as the AFM should be a horizontal line......

Had massive issues with mine, as it doesn't seem to want lay flat and overfuels a hell of a lot.

Steve

I know you have had a few issues with this on your car but just looking at the graph, the AFR is 14 when it starts to misfire which is weaker than the 13.2 to 12.6 to 1 that B-Reight mentioned it should be. I am just not sure whether that difference is enough to make it misfire under load.

In terms of a new AFM even a reconditioned one is £300 inc VAT from Power on an exchange basis; it is difficult to see what you could do to an AFM for that kind of money. By the way, on your 4.3 is it a 5AM or a 3AM that you have fitted, mine is a 3AM (it is written on the black plastic bit towards the top). I need to do a bit of research and find out what the difference is.

[grngriff]

Hi

My 4.3 has a 5am afm, got a spare one if any help to you mate.

Pete

[FactBV]

grngriff said:

Hi

My 4.3 has a 5am afm, got a spare one if any help to you mate.

Pete

That is very kind of you, thank you for the offer; I might take you up on that depending on how I get on with the fault finding. Incidentally, any idea what the difference is between a 3M and a 5M? Mine is a 4.3BV but with a kent 224 cam which is a higher lift cam compared to the original. Its not been on a rolling road since it was fitted 5 years ago. If a 5M is larger than a 3M, I wonder if it needs it?

Thanks again. Peter

[HarryW]

I thought the main difference was the scaling of the output, 3v and 5v peaks. Not sure what the effects would be if you have the wrong AFM for the ECU map and vica versa.

[blitzracing]

This is crazy- I cant ever think of an AFM doing this short of thing on a linear scale- having said that a failing AFM can make the engine run very rich low down, and it is the major fueling component. At idle you should see 1.6 Volts (3.5 Litre engine) to 1.75 Volts (5.0 Litre engine). My 3.9 runs at 2.62 volts, so inspects you to be just a bit higher. Do this basic DC check before parting with any money to swap AFMs. A failed AFM will be out by a long way out to make it run that rich. Has the CO trim been set up correctly with the pot on the side of the AFM? This will skew the readings up to around 2400 rpm if its set wrongly. BTW The Lucas 3AM and 5 Am are the same units, both scale to 5 volts at the same level of airflow to voltage readings.


Also check your fuel map- open up the ECU and see whats written on the chip. People love to swap chips thinking it will release power, when it does not, it just screws the fueling. If the car was originally fitted with catalysts, the original chip will have something like 4.2 cat written on it. This chip is unlikely be suitable for non cat as its likely to be a Range Rover tune, as TVR did not modify both cat and non cat fuel maps for given engine applications far as I can see looking at the physical maps.

Edited by blitzracing on Sunday 9th June 18:33

Edited by blitzracing on Sunday 9th June 18:36

[grngriff]

Just looked it up, the 3am are fitted to non cat cars and the 5am to the later catted cars.
That would explain why mine has the 5am as it use to be 4.3 catted, now 4.3 BV non cat.
So yours should be fine if working ok.

Pete

[blitzracing]

grngriff said:

Just looked it up, the 3am are fitted to non cat cars and the 5am to the later catted cars.
That would explain why mine has the 5am as it use to be 4.3 catted, now 4.3 BV non cat.
So yours should be fine if working ok.

Pete

Its simply a case of who made the AFM- the 3 AM is an Hitachi design used in the early 1990's, to be replaced with Lucas's own 5AM. Totally plug compatible, with or without catalysts.

[FactBV]

blitzracing said:

This is crazy- I cant ever think of an AFM doing this short of thing on a linear scale- having said that a failing AFM can make the engine run very rich low down, and it is the major fueling component. At idle you should see 1.6 Volts (3.5 Litre engine) to 1.75 Volts (5.0 Litre engine). My 3.9 runs at 2.62 volts, so inspects you to be just a bit higher. Do this basic DC check before parting with any money to swap AFMs. A failed AFM will be out by a long way out to make it run that rich. Has the CO trim been set up correctly with the pot on the side of the AFM? This will skew the readings up to around 2400 rpm if its set wrongly. BTW The Lucas 3AM and 5 Am are the same units, both scale to 5 volts at the same level of airflow to voltage readings.


Also check your fuel map- open up the ECU and see whats written on the chip. People love to swap chips thinking it will release power, when it does not, it just screws the fueling. If the car was originally fitted with catalysts, the original chip will have something like 4.2 cat written on it. This chip is unlikely be suitable for non cat as its likely to be a Range Rover tune, as TVR did not modify both cat and non cat fuel maps for given engine applications far as I can see looking at the physical maps.

Edited by blitzracing on Sunday 9th June 18:33

Edited by blitzracing on Sunday 9th June 18:36

Thanks for all the pointers, I will look at those. Just looking at the graph; we have realised that the linear line at 45 degrees is the power line rather than the AFR. The mixture is the flatter line that goes from 13:1 at around 1500rpm, to 14.8:1 at 2,800 rpm back to 14:1 at 4800 rpm where it misfires. As the AFR is higher, does that not mean that it is running lean, not rich? If it is lean, is it lean enough to result in a misfire regardless of what is causing it for the moment. Thanks in advance. Peter

[blitzracing]

14:1 is somewhat lean at high rpm under load, but I cant say if it would misfire. Its worth bearing in mind that if the mixture is measured by measuring the exhaust oxygen content, (a lambda probe) both a lean mixture or a misfire will show up as high oxygen- hence a lean reading. Misfires should show up as high hydrocarbons (unburnt fuel) but I dont know how the exhaust would have been measured, you need to ask the people who did the test.

[FactBV]

blitzracing said:

14:1 is somewhat lean at high rpm under load, but I cant say if it would misfire. Its worth bearing in mind that if the mixture is measured by measuring the exhaust oxygen content, (a lambda probe) both a lean mixture or a misfire will show up as high oxygen- hence a lean reading. Misfires should show up as high hydrocarbons (unburnt fuel) but I dont know how the exhaust would have been measured, you need to ask the people who did the test.

Thanks, I plan to give Surrey Rolling Road a call first thing in the morning, it was a bit busy on Saturday with all the other cars.

[900T-R]

Am I crazy or is the 45ish degree diagonal line the power output (which would correspond with the 239 bhp peak @ 4.7K rpm) and the other one the AFR?

- In which case isn't it simply a matter of the fueling not being able to keep up for some reason (fuel pump, blocked fuel filter?) ?

[FactBV]

900T-R said:

Am I crazy or is the 45ish degree diagonal line the power output (which would correspond with the 239 bhp peak @ 4.7K rpm) and the other one the AFR?

- In which case isn't it simply a matter of the fueling not being able to keep up for some reason (fuel pump, blocked fuel filter?) ?

Yes indeed, the 45 degree line is definitely the power line. I hadn't thought of the fuel supply side but I guess that it could also just be the coil breaking down, other ignition problems etc. As the only fault showing was a 'faulty map tune resistor' from the ECU fault codes, I was hoping that there might be an obvious problem linked to that. I need to do some research here starting with a chat about the print out with Surrey Rolling Road. Thanks for your input.

[DarkMatter]

I'm not sure whether my experience of a high Air to Fuel ratio on my Griffith 500 will be relevant to the problem you are experiencing but here goes anyway.....

I spent a lot of time measuring voltages, of various components, either at the component or at the ECU connector. Eventually I homed in on the voltages relating to the throttle pot. The throttle pot has 3 wires which connect back to the ECU at pins 3 (yellow), 25 (red/black), and 20 (red). As I recall the voltage between pin 20 and 25 should range from 0.26/0.36v with throttle closed to 4.6/4.9v with throttle fully open but on my car with the throttle fully open I consistently measured only 3.9v - note that this was done by removing the throttle pot from the throttle body and turning it by hand.

I fitted a new ECU and this resulted in the correct voltage of 4.9v between pins 20 and 25 at full throttle and more importantly the air fuel ratio was correct.

The following chart shows the AFR before (green line) and after (red line) I resolved the problem.
.

My discussion thread is "here" .

[FactBV]

DarkMatter said:

I'm not sure whether my experience of a high Air to Fuel ratio on my Griffith 500 will be relevant to the problem you are experiencing but here goes anyway.....

I spent a lot of time measuring voltages, of various components, either at the component or at the ECU connector. Eventually I homed in on the voltages relating to the throttle pot. The throttle pot has 3 wires which connect back to the ECU at pins 3 (yellow), 25 (red/black), and 20 (red). As I recall the voltage between pin 20 and 25 should range from 0.26/0.36v with throttle closed to 4.6/4.9v with throttle fully open but on my car with the throttle fully open I consistently measured only 3.9v - note that this was done by removing the throttle pot from the throttle body and turning it by hand.

I fitted a new ECU and this resulted in the correct voltage of 4.9v between pins 20 and 25 at full throttle and more importantly the air fuel ratio was correct.

The following chart shows the AFR before (green line) and after (red line) I resolved the problem.
.


My discussion thread is "here" .

Thanks very much, what this does show is that I am going to have to go through this very methodically. Lets hope that it is something simple!