Diagnose this......!
Discussion
Following my engine rebuild earlier this year, I've been trying to fine-tune the 390, ideally for a little more economy ;-) but mainly to cure a few running glitches and idle unevenness.
The CO has been around 3.7%, best I could achieve regardless of adjustmnt of screw in airflow meter (it's all the way up!). Idle speed is 800 rpm.
ECU coolant temp sensor is new, throttle pot has been rewired and believed OK, ouputs 310 mV at idle, increasing progressively with throttle opening. Spraying lighter fuel or similar fails to reveal any air leaks. Ignition has been around 10 deg BTDC.
Coolant temp is fairly steady at about 88 degrees, will rise under hard use but generally corrects itself.
Fuel pressure measures 3.6 bar, reducing as vacuum is applied to the regulator. With the engine off the fuel rails hold 2.9 bar.
If you hang a scope on the injectors and rev the engine you can see the trigger pulse width varying.
Now, I've advanced the timing off the end of the scale (the TDC mark is correct, I checked when the engine was in bits) and the CO is down to about 2.7%. Idle is now a little erratic. Car pulls like a train, except that under hard acceleration you can feel a 'surging' effect. Under cruise conditions the temp gauge reads lower than it was at 10 deg timing! I can lower the CO further by increasing the advance but then the idle is atrocious and you can hear pinking under moderate acceleration.
So, what's going on? Do I need moe advance, less advance; is the camshaft knackered although it was fine 2000 miles ago? If the 10 degree setting was OK then why the high CO? Technically it's high enough to fail an MOT, yet the engine is broadly (to my eyes and ears) OK.
Are we looking at a rolling road session to fettle this, or am I missing something?
Baffled of Teesside.
The CO has been around 3.7%, best I could achieve regardless of adjustmnt of screw in airflow meter (it's all the way up!). Idle speed is 800 rpm.
ECU coolant temp sensor is new, throttle pot has been rewired and believed OK, ouputs 310 mV at idle, increasing progressively with throttle opening. Spraying lighter fuel or similar fails to reveal any air leaks. Ignition has been around 10 deg BTDC.
Coolant temp is fairly steady at about 88 degrees, will rise under hard use but generally corrects itself.
Fuel pressure measures 3.6 bar, reducing as vacuum is applied to the regulator. With the engine off the fuel rails hold 2.9 bar.
If you hang a scope on the injectors and rev the engine you can see the trigger pulse width varying.
Now, I've advanced the timing off the end of the scale (the TDC mark is correct, I checked when the engine was in bits) and the CO is down to about 2.7%. Idle is now a little erratic. Car pulls like a train, except that under hard acceleration you can feel a 'surging' effect. Under cruise conditions the temp gauge reads lower than it was at 10 deg timing! I can lower the CO further by increasing the advance but then the idle is atrocious and you can hear pinking under moderate acceleration.
So, what's going on? Do I need moe advance, less advance; is the camshaft knackered although it was fine 2000 miles ago? If the 10 degree setting was OK then why the high CO? Technically it's high enough to fail an MOT, yet the engine is broadly (to my eyes and ears) OK.
Are we looking at a rolling road session to fettle this, or am I missing something?
Baffled of Teesside.
Guess 3.7% CO is at idle speed ?
Otherwise 3.7 sound just perfect !
Ok, lowering fuel pressure slightly will lower the CO but will also remove some power from the engine.
- It may be far-fetched but in the air flow meter there is a flapper bump stop, at the air filter side, that will bend after some years of stopping the flapper. It will bend inwards and you can remove it and bend it back so that the flapper covers the hole without any "chink" again. Maybee it may affect the pot inside the air flow meter...?
bengt
Otherwise 3.7 sound just perfect !
Ok, lowering fuel pressure slightly will lower the CO but will also remove some power from the engine.
- It may be far-fetched but in the air flow meter there is a flapper bump stop, at the air filter side, that will bend after some years of stopping the flapper. It will bend inwards and you can remove it and bend it back so that the flapper covers the hole without any "chink" again. Maybee it may affect the pot inside the air flow meter...?
bengt
OK< quick update:
1. Plugs are OK. However they have done some thousands of miles, maybe time for replacement.
2. Yes, 3.7% is at idle. It goes up when you rev the engine, then reduces again, as you'd expect it to, at constant throttle.
3. Yes, the vac advance is fine, and the timing was set with it disconnected.
4. A defect with the airflow meter is possible, as the car has already had a reconditioned one (from Wedge Auto) fitted at some point prior to my ownership.
5. Yesterday, the car ran OK to start with on a trip to a DIY shop. When I came back out, though, the car was running rough, as if out of fuel. More fuel didn't cure it. Left it to stand for a few hours, and then seemed to be running OK again, when cold. Maybe there's some bizarre thermal effect going on here?
6. Tried the CO meter on another car; this showed 0.5% CO so I don't think the meter is telling me lies.
Curiouser and curiouser.
Ian
1. Plugs are OK. However they have done some thousands of miles, maybe time for replacement.
2. Yes, 3.7% is at idle. It goes up when you rev the engine, then reduces again, as you'd expect it to, at constant throttle.
3. Yes, the vac advance is fine, and the timing was set with it disconnected.
4. A defect with the airflow meter is possible, as the car has already had a reconditioned one (from Wedge Auto) fitted at some point prior to my ownership.
5. Yesterday, the car ran OK to start with on a trip to a DIY shop. When I came back out, though, the car was running rough, as if out of fuel. More fuel didn't cure it. Left it to stand for a few hours, and then seemed to be running OK again, when cold. Maybe there's some bizarre thermal effect going on here?
6. Tried the CO meter on another car; this showed 0.5% CO so I don't think the meter is telling me lies.
Curiouser and curiouser.
Ian
Ok, sorry about my English...
If the flapper "bump stop" is bent over time, the flapper will not cover the inlet 100% at idle, also the internal meter will change position. A small gap will be the result and the engine will start sucking air through the flapper/meter at idle rpm instead of the bypass channel(which is adjustible) Not shure if it really matters, but the rubber/metal bump stop really bends by time. Mine had a visible gap after 60000km, so a friends. Just check it, doesn´t cost a euro... But since you have e rebuilt one it my not be the case here.
As Danny suggested, try turn down the pressure. I´m running 2,8bar with 1,6% CO at 12deg stat advance.. Gives about 4,5% while hitting the floor
. Takes a couple of minutes if you have one installed to adjust. Probably works as a solution but you have a 390 so 2,8 might be low...
bengt
If the flapper "bump stop" is bent over time, the flapper will not cover the inlet 100% at idle, also the internal meter will change position. A small gap will be the result and the engine will start sucking air through the flapper/meter at idle rpm instead of the bypass channel(which is adjustible) Not shure if it really matters, but the rubber/metal bump stop really bends by time. Mine had a visible gap after 60000km, so a friends. Just check it, doesn´t cost a euro... But since you have e rebuilt one it my not be the case here.
As Danny suggested, try turn down the pressure. I´m running 2,8bar with 1,6% CO at 12deg stat advance.. Gives about 4,5% while hitting the floor
. Takes a couple of minutes if you have one installed to adjust. Probably works as a solution but you have a 390 so 2,8 might be low... bengt
Danny:
I take your point about the plugs. They do appear more or less the right colour, although there was some sooting after the rebuild due, I presumed, to oil in the combustion chambers. Now it's done a couple of thou maybe I should check them again! Coincidentally, I was going to nip to Halfords to source some new plugs using some spare gift vouchers that I've had for a while, but seeing you had trouble I thought I might not bother!
Bengt: I will certainly check the airflow meter as you suggest. Anything is worth trying. This is one thing that I think the internet is useful for: you can find a lot of information very quickly, even if sometimes it is the wrong information!
Ian
I take your point about the plugs. They do appear more or less the right colour, although there was some sooting after the rebuild due, I presumed, to oil in the combustion chambers. Now it's done a couple of thou maybe I should check them again! Coincidentally, I was going to nip to Halfords to source some new plugs using some spare gift vouchers that I've had for a while, but seeing you had trouble I thought I might not bother!
Bengt: I will certainly check the airflow meter as you suggest. Anything is worth trying. This is one thing that I think the internet is useful for: you can find a lot of information very quickly, even if sometimes it is the wrong information!
Ian
Try a little more advance +4° say and close the throttle butterfly down to achieve the idle speed you want. If this works OK without Pinking then it may be time to have the dizzy rebuilt by someone like Aldon Automotive alternativly you could upgrade to mapped ignition and definitley get more efficiency then. however the 300 quid or so it would cost does buy quite a bit of petrol.... 
Matt
Matt
Just a thought. Lifting the black plastic cover of the air flow meter will expose the resistance plate with the contact arm resting on it. With the engine off the contact arm is against the stop. With the engine ticking over the conatct arm will be between an 1/8 and a 1/4 of its full allowable travel so I don't think adjusting the bump stop will have any effect.I suspect that the meter was set for standard pressure of about 2 bar and if you have it set higher, which you need to do, then it will affect CO. Also the cam you have fitted can have an effect on the CO. You could try increasing the spring pressure applied to the contact arm a few notches at a time. I have done this with my mildly tuned 350 which I run at 2.5 bar to get the CO in the region of 2%. Increasing the spring pressure slightly shouldn't affect full throttle operation.
Yup it does have a non-standard cam fitted (RPI Eng. RP1). Last night I did the old trick of lifting the bonnet in the dark to watch for HT leaks... and found some!
The coil is dated 1986 and is thus quite likely to be the original. There's a good degree of carbon inside the HT cone, and I think HT was leaking up to the rubber cap, down the outside and jumping to the LT wiring. Cured that with silicon grease and a good clean, then found that one of the HT lead clamps was sparking to the rocker cover. Removing the clamp stopped that, but it does seem likely that the HT system as a whole could do with an overhaul. Funny how you think these things are alright, isn't it?! I've just ripped one of the HT leads out of a plug cap without too much effort, so tonight is blitz the ignition time. Might throw up some more defects, might not.
There still seems to be a temp-related effect going on insofar as the rough running is more noticeable when the engine's warmed up. I meant to say earlier that the cold start injector has been removed, so overfuelling (if that be the trouble) is not being caused by that. I've been pricing up the relevant bits to do a complete rewire of the injection system; probably about 70-80 quid but at least it removes a bit of uncertainty! I was thinking of including a diagnostics box to give easier access to things like the injector pulses, throttle pot voltage etc. for future fault-finding.
More expense.....;-)
Ian
The coil is dated 1986 and is thus quite likely to be the original. There's a good degree of carbon inside the HT cone, and I think HT was leaking up to the rubber cap, down the outside and jumping to the LT wiring. Cured that with silicon grease and a good clean, then found that one of the HT lead clamps was sparking to the rocker cover. Removing the clamp stopped that, but it does seem likely that the HT system as a whole could do with an overhaul. Funny how you think these things are alright, isn't it?! I've just ripped one of the HT leads out of a plug cap without too much effort, so tonight is blitz the ignition time. Might throw up some more defects, might not.
There still seems to be a temp-related effect going on insofar as the rough running is more noticeable when the engine's warmed up. I meant to say earlier that the cold start injector has been removed, so overfuelling (if that be the trouble) is not being caused by that. I've been pricing up the relevant bits to do a complete rewire of the injection system; probably about 70-80 quid but at least it removes a bit of uncertainty! I was thinking of including a diagnostics box to give easier access to things like the injector pulses, throttle pot voltage etc. for future fault-finding.
More expense.....;-)
Ian
When the outside temperature exceeds 20C, light aircraft are forbidden to fly, when burning Mogas, 4 star to meer mortals. As your engine compartment will reach very high temperatures, particularly after a run, heat soaking of fuel line and components can inevitably lead to vapour locking. When I used to fly aircraft buring Mogas, I always removed the engine cowlings to allow the heat to escape. Another boring fact is that petrol companies modify the brew of their products in colder months, to aid in cold starting, so if your car has been off the road since the winter, and still has a take of winter fuel, vapour locking is even more likely. On the other hand it may be nothing to do with vapour at all...
Lee
Lee
Off the road since winter??!! How dare you, sir! I'll have you know my TVR is almost as well-used as my ex-wife. Both of them in fact... ;-)
Still haven't entirely got to the bottom of this, although replacing the HT leads knocked 0.5% off the CO figure. Plugs are next on the list, but first I have to take me Julie to St. Tropez next week, and I think she'll drink more than a set of NGK's finest will set me back....
Ian
Still haven't entirely got to the bottom of this, although replacing the HT leads knocked 0.5% off the CO figure. Plugs are next on the list, but first I have to take me Julie to St. Tropez next week, and I think she'll drink more than a set of NGK's finest will set me back....
Ian
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