Checking the hotwire afm

sowen · Post by **sowen** » Tue Jul 15, 2014 12:02 pm

Are there any specific guides on checking the operation of the hotwire airflow meter?

I suspect the one I have on my SD1 isn't reading correctly, the security plug has been opened and it has the usual scrapyard marker pen on it!

The engine is an SD1 3.5 on hotwire with a 3.5 Discovery ecu, and seems to be running a little rich. Removing a blanking plug on the side of the plenum t give a 4mm hole got the emissions clean through the test and drove fine on the way home. I'm fairly certain the coolant sensor is operating (how accurate I can't say) as removing the plug with the engine running richens it further.

Ian Anderson · Post by **Ian Anderson** » Tue Jul 15, 2014 12:25 pm

I was not aware they put hotwire on the 3.5 - those were all Flapper from what I read.
From the 3.9 in 1990 they changed to hotwire

But I am sure some with more knowledge will prove me wrong

Ian

sowen · Post by **sowen** » Tue Jul 15, 2014 2:18 pm

Ian Anderson wrote:I was not aware they put hotwire on the 3.5 - those were all Flapper from what I read.
From the 3.9 in 1990 they changed to hotwire

But I am sure some with more knowledge will prove me wrong

Ian

You are correct in the SD1 originally had the flapper system. My car is orignally a carb model, but I'm just finishing off retrofitting a hotwire system from a 3.9 Discovery, using an early 3.5 Discovery ecu

I've a fair bit more work to do to finish it, some brackets and final setting up mainly.

DaveEFI · Post by **DaveEFI** » Tue Jul 15, 2014 3:16 pm

I don't think I've heard of a 3.9 hotwire system being fitted to a 3.5 before.
Plenty of 3.9 engines with hotwire in an SD1, though.

It's obviously going to need the fuel delivery altered from standard in some way. Good luck.

sowen · Post by **sowen** » Tue Jul 15, 2014 6:06 pm

DaveEFI wrote:I don't think I've heard of a 3.9 hotwire system being fitted to a 3.5 before.
Plenty of 3.9 engines with hotwire in an SD1, though.

It's obviously going to need the fuel delivery altered from standard in some way. Good luck.

According to my Discovery parts catalogue I've got the correct early ecu, PRC9059, which is Discovery 3.5efi, so apart from an age difference, it should in theory be to factory specifications as far as I can tell? There is a difference in afm later in the Discovery production, but I can't see any other differences that correspond to the change in capacity from 3.5 to 3.9.

Ian Anderson · Post by **Ian Anderson** » Tue Jul 15, 2014 6:29 pm

What tune resistor are you running?

One says ignore the lambda sensors so it runs richer to be safe for the engine

The other sniffs the exhaust and tries to get it to specifications

Ian

sowen · Post by **sowen** » Tue Jul 15, 2014 6:42 pm

Ian Anderson wrote:What tune resistor are you running?

One says ignore the lambda sensors so it runs richer to be safe for the engine

The other sniffs the exhaust and tries to get it to specifications

Ian

It was originally the white tune resistor, I cut that off and soldered a 470ohm resistor in it's place to replicate the green tune resistor. The plugs are still in place on the loom for lambda's so I could if I wanted to get a pair and fit them to my downpipes. I really did notice a difference going from one resistor to the other in the way the engine ran, and the richness from the exhaust.

I bought a complete 3.9v8 from a Discovery, and I found that when I took the front cover off the afm to turn the outer circumference down to take a straight air filter/hose there was some muddy/oily debris in the bottom. My suspicion is that there is a possibility that it had possibly ingested some muddy water?

Spongo · Post by **Spongo** » Tue Jul 15, 2014 9:54 pm

Hotwire AFM's dont like water mud etc, been there done that

try to unplug it and have a drive I think that should prove if it's faulty as the car should run reasonably OK without it, the idle might be a bit high thats all as far as I can remember.

sowen · Post by **sowen** » Wed Jul 16, 2014 7:35 am

Cheers for that, I'll give it a go. I did unplug the afm briefly when first running the engine and the idle speed got worse, with it plugged in there is a slight hunting, possibly about 50rpm or so, but most of the time it's fairly stable.

richardpope50 · Post by **richardpope50** » Fri Jul 18, 2014 10:53 am

If you are interested, I have a Hotwire AFM that I no longer need. It was running fine before I switched to a MS system. came from my 44,000 mile TVR donor (so I have the ECU too but that's the latest 14CUX)..

landrovernuts · Post by **landrovernuts** » Sat Jul 19, 2014 6:39 am

I can confirm some early discoverys were fitted with 3.5 engines with hotwire injection. I have a complete engine and injection system which I have raided for parts and the ECU part number is PRC 9059 which is for a 3.5 engine. I have used this ECU for fault finding when I had issues with cold idle a while ago and my current engine (4.2) ran very well with it.

Toby

sowen · Post by **sowen** » Sun Jul 20, 2014 7:27 pm

richardpope50 wrote:If you are interested, I have a Hotwire AFM that I no longer need. It was running fine before I switched to a MS system. came from my 44,000 mile TVR donor (so I have the ECU too but that's the latest 14CUX)..

You have PM

landrovernuts wrote:I can confirm some early discoverys were fitted with 3.5 engines with hotwire injection. I have a complete engine and injection system which I have raided for parts and the ECU part number is PRC 9059 which is for a 3.5 engine. I have used this ECU for fault finding when I had issues with cold idle a while ago and my current engine (4.2) ran very well with it.

Toby

I've read of the ecu's working on different capacity engines, just maybe not quite the right fuel map?

I haven't had a chance to take the car out again, but did take the plugs out to replace. The old plugs looked pretty clean, so maybe it is running better than I thought and just needs the cobwebs blowing out

landrovernuts · Post by **landrovernuts** » Mon Jul 28, 2014 1:56 pm

You are correct the fuel map would have been well out, which is why I was supprised it ran so well! I put the 4.2 ECU back shortly after as I was just trying to sort another problem out which turned out to be air flow meter issues in the end (one that come in a blue box and was useless).

Toby

SimpleSimon · Post by **SimpleSimon** » Mon Jul 28, 2014 9:15 pm

landrovernuts wrote:I can confirm some early discoverys were fitted with 3.5 engines with hotwire injection. I have a complete engine and injection system which I have raided for parts and the ECU part number is PRC 9059 which is for a 3.5 engine. I have used this ECU for fault finding when I had issues with cold idle a while ago and my current engine (4.2) ran very well with it.

Toby

Yes they did for certain, my sister & bro-law had a 1 owner 3.5i RV8 Disco with man transmission on Lucas Hotwire it was the 1st time I was itroduced to a sticky stepper motor back in the nineties (stalling at junctions)

sowen · Post by **sowen** » Sun Aug 03, 2014 9:29 pm

I've done over a hundred miles of enthusiastic driving in my SD1 with the hotwire and the plugs are clean, no black deposits on the tips like I was having before

The afm was swapped with the one I bought from Richard, and it made a small noticeable difference to the idle and low rpm smoothness. If I can I'll try to book it into my local garage to check the emissions, and do some mpg calculations to see how well it's running now