Old Classic's electrics

Anyone any ideas as to what would cause 1971 classic to develop a bit of a misfire once it's hot and been running about ten miles? It had a 3.9 EFi conversion a few years back but sat unused for the last three years before coming back on the road (free tax and all that ). I have cleaned the spark plugs and changed the coil but the other day it happened again then cleared during the same run. The wiper's wouldn't work the other night so I gave the fuses a tweak and one of them sparked and on they came. There are only three fuses in the box so they are obviously shared by several circuits.

This seems to be a regular question on here.

You need to identify if the problem is an ignition misfire or an injection misfire. The ignition system is pretty simple and inlikely to be the problem, however here are some pointers.

Check vacuum advance is working, check mechanical advance is working, check your timing chain is not slack as nicker elastic. Replace/swap out your ignition amplifier (intermota do one at sensible money). Check if you have a ballast resistor, also replace capacitor on the coil. That pretty much covers the ignition side.

If the problem is on the fuel injection it gets a bit more involved. Air flow meter, temperature thermistor, fuel thermistor, throttle potentiometer - all the sensors basically - are all inter-connected, you should check/test each one. Check fuel filter, pressure regulator and fuel pump. If everything so far is where it needs to be then you need to start thinking it may be the ecu.

On my 3.9, the original ecu wasn't reading the air flow meter. I could get it to run but only by disconnecting the AFM, expect 6mpg in limp home mode, but it is an excuse to bury your foot in the carpet everywhere!! I replaced the ecu and AFM (AFM was not the problem) but the problem wasn't fixed, it was a bit better but I had to look elsewhere. The next thing to look at is your alternator - for what it costs, get the thing overhauled - voltage regulator, brushes etc.

If that doesn't work - then you really have got a mystery. Start to check individual injectors and don't forget the air bypass valve. Check all vacuum hoses.

Sorry if this is a bit long winded but diagnosing a misfire on one of these isn't exactly straight forward. I know, it took me 12 months to get mine running properly. Good luck, don't give up on it.

BTW, if it is alternator related, try seeing how bad it is when you put on main beam, heated rear screen and heater fan. If this makes it really bad or even stop - it's your alternator.

Thanks, plenty to go on there. Easiest one will be to load up the alternator once it does it again and then work back through the big list. Thinking back I had a similar prob with a V6 Sierra years ago and it was the potentiometer.

hippolover said:

Thanks, plenty to go on there. Easiest one will be to load up the alternator once it does it again and then work back through the big list. Thinking back I had a similar prob with a V6 Sierra years ago and it was the potentiometer.

Click to expand...

No problem. One other thought, I used to get misfire when engine up to temperature and giving it the beans, back throttle off, misfire would disappear - never got to the bottom of that as I suspect there were two or three gremlins having a party under the bonnet.

Got 'em all in the end though

Had a similar thing years ago, turned out to be one of the hoses. The jubilee clip was sitting pretty but not tightened enough to be doing it's job.
Consequently when it got hot it softened and allowed air in.
Not sure which one it was (it died whilst getting out of a bog so didn't pay too much attention) - but it's certainly worth a look and check any hoses & clips that might be to do with the advance or air intake through various parts of the system.

Hello Hippo,

Being an old car its liable to suffer elecrical connection problems and even the Wiring loom on your transplanted Efi system has the same issues.

So check all your ignition and fuel injection electrical connections.

The majority of Rover Efi faults are caused by connector problems and rogue airleaks into the plenum chamber.

I cant see mention of what type of Efi system you have - flapper or hot wire -but quite a few of the components are the same and also similar to the Rover SD1 flapper system.

You can read all about that system from this index on my website which also defines how the component tests are to be carried out.

Rover SD1 Efi System Components

It may be of help to you, particularly the Plenum essay which defines a comprehensive Air Leak test process.

Elsewhere on the site is a test program that encompasses the system as a whole.

Ramon

Vintage Model Airplane and Rover SD1 3500cc Twin Plenum Vitesse

Changed the air filter today as the old one was seven shades of grey (kind of like the sky in Scotland this summer) and now it idles about 2000 revs. Hand over the intake trumpet and the revs drop. Methinks the whole thing has previously been set up with the old, fouled bits and now needs reset. Only prob now is I can't find an idle adjuster screw. The throttle linkage stops against a little nub that pokes up through the edge of the plenum chamber. If that is threaded it is certainly an awkward place to try and fit a screwdriver. Any help greatly appreciated.

hippolover said:

Changed the air filter today as the old one was seven shades of grey (kind of like the sky in Scotland this summer) and now it idles about 2000 revs. Hand over the intake trumpet and the revs drop. Methinks the whole thing has previously been set up with the old, fouled bits and now needs reset. Only prob now is I can't find an idle adjuster screw. The throttle linkage stops against a little nub that pokes up through the edge of the plenum chamber. If that is threaded it is certainly an awkward place to try and fit a screwdriver. Any help greatly appreciated.

Click to expand...

O.K. so we're talking about a 3.9 efi, so probably hot wire system. Sounds like you need to reset your base idle, it might also be worthwhile checking your CO trim.

Hope you don't mind a bit of a long read -
Setting the base idle is done by first driving the vehicle for a couple of miles and getting it up to normal temperature.

Turn everything electrical off, including A/C if you have it.

Remove the air bypass hose and blank off/seal where it is normally connected. I just sealed one end of the pipe and the "open" connection.

Start the engine and adjust the idle speed to 525rpm + or - 25rpm, the adjustment is made turning the idle screw on the plenum housing, near where the flexi house from AFM connects, there may be a tamper proof plug you need to remove but if not just put the right size allen key in there and give the screw a twiddle, one way will increase revs the other decrease.

Once you have set base idle, switch off engine, reconnect air by pass hose, fire her up and hopefully the ecu will tell the air bypass valve (stepper motor) to do its' thing and put the idle speed up to roughly 700rpm. Simple eh

Now you might want to check your CO trim, that's the screw on the Air Flow Meter, again possibly under a tamper proof plug (if it's still there). What you'll need to do is get a Digital Volt Meter (DVM). Carefully pull back the rubber cover on the plug that is connected to the AFM, you are looking to get a reading of between 1.0v and 1.5v measured across the sensor ground and CO trim.

The wires going into the AFM are as follows:

Red with Black - Sensor Ground
Blue with Green - Air Flow Signal
Brown with Orange - Ign live +12v
Blue with Red - CO Trim

If you're interested the Air Flow Signal and CO trim inputs to the 14 CUX ECU correspond to pins 35 and 22 respectively.

I think I set my CO trim to about 1.25v, I seem to remember there is more data about this on the RPI Engineering website.

Don't forget to check/re-check your ignition timing and make sure there are no vacuum leaks - these could affect setting the base idle speed as well as making sure the engine is up to temperature before you set anything.

The other thing you might want to consider is disconnecting the ECU for a couple of minutes then plug it back in, turn the ignition to "on" listen for the relays switching, then fire the engine, let it idle for a minute or two and see if the idle comes down, if not, drop it in gear and see if that settles it down.

The usual disclaimers apply here, as ou'll appreciate without seeing the engine and wiring I can only make suggestions based on what would be standard, but I hope that helps.

Plenty things to try this w/e then. I'll let you know how it goes. One obvious question though, which is the air bypass hose?

hippolover said:

Plenty things to try this w/e then. I'll let you know how it goes. One obvious question though, which is the air bypass hose?

Click to expand...

Quite easy, its the hose at the back of the plenum that connects next to the air bypass valve (stepper motor) and runs round just in front of the throttle linkage.

Update - Before getting a chance to do all the checks the idle settled down to a more acceptable figure (prob about 800 - no rev counter fitted). Could this have been the ecu relearning its values after being run for a few miles? Also I checked the alternator output the other day and, irrespective of the revs, it reads 12.7V. Regulator??

hippolover said:

Update - Before getting a chance to do all the checks the idle settled down to a more acceptable figure (prob about 800 - no rev counter fitted). Could this have been the ecu relearning its values after being run for a few miles? Also I checked the alternator output the other day and, irrespective of the revs, it reads 12.7V. Regulator??

Click to expand...

Almost certainly. Get the alternator seen to - regulator, slip rings and brushes. You may find this will help with the idle speed too, I seem to remember reading somewhere that idle speed will pick up if voltage is low. You should be getting about 14v at idle, say somewhere between 13.8v and 14.4v. This will affect the running of the car as the alternator triggers the injection pulse.

classic kev said:

You should be getting about 14v at idle, say somewhere between 13.8v and 14.4v. This will affect the running of the car as the alternator triggers the injection pulse.

Click to expand...

Not too sure that low voltage will affect the way the engine runs although it will affect the injection pulsewidth.

Part of the ECU pulsewidth calculation uses VBat so that it can compensate for the varying injector opening times caused by varying battery voltage. It does this by automatically adjusting the injector pulsewidth to compensate, the whole purpose of this is to ensure that battery voltage does not affect the amount of fuel delivered.

Something still needs sorting tho' as 12.7v at idle is normally no good. The only time you'll see 12.7v at idle is if your battery was pretty flat and in the process of being charged.

The old beast is still playing up. May have to admit defeat and get local LR independent specialist to take a peek. (It'll be fun watching him look for the OBD socket though!!)

hippolover said:

The old beast is still playing up. May have to admit defeat and get local LR independent specialist to take a peek. (It'll be fun watching him look for the OBD socket though!!)

Click to expand...

Would be interested to learn what they think has caused the problem and whether or not they manage to fix it.