High rpm misfire under load

[sowen]

This has been an ongoing issue I've had with my SD1 since I got it, as the revs go over 4000 it begins to sound like it's misfiring under the bonnet, peaking around 5000rpm. I was out and about earlier and held it in gear for a little longer and it seemed to pull through, not sure of what rpm the miss quietened down at?

Since getting my SD1, I've had it running on carbs, hotwire efi and now Megasquirt, the ignition system has changed from the original dizzy to EDIS8, and the problem has persisted exactly the same. Earlier today with the help of my dad we re-checked the ignition timing, did a few runs with different advances to rule out detonation, and done a compression test which gave 135-150psi on all 8 cylinders.

Another more recent problem is there's a light top end tapping at low rpm, with the engine at operating temperature, generally at light load cruising. Backing off or pressing the throttle makes it go away, sounds like one of the lifters has lost pressure and it does it most of the time. I'm not sure if this is related to the high rpm misfire problem? I have been wondering if it's one of the rods as it does it generally only at cruising loads?

My dad suggested the high rpm misfire could be some of the valves floating, a quick Google search does show very similar symptoms, as the engine has always felt flat at higher rpms, it doesn't pull hard like my old Land Rover did. Low to mid rpm it's fine, just lacks the ooomph when the throttle is held wide open

Any thoughts and suggestions before I go and tear the top end apart?

[820]

These engines can suffer with camshaft wear, the lobes wear flat and reduce valve lift which generally causes a complete lack of power but would be more noticeable at higher RPM.
What mileage has the engine done, do you know its history, if I remember correctly you can see the camshaft with the inlet manifold removed.

[sowen]

May 3, 2015 18:09:45 GMT 820 said:

These engines can suffer with camshaft wear, the lobes wear flat and reduce valve lift which generally causes a complete lack of power but would be more noticeable at higher RPM.
What mileage has the engine done, do you know its history, if I remember correctly you can see the camshaft with the inlet manifold removed.

I'm fairly certain that when I swapped the carbs over to efi I had a look at the camshaft and the lobes were still lobey, not flat and rounded. Engine has done about 100k miles, apart from the noises and oil leaks everything has visibly been ok on it so far...

[colnerov]

Hi, it could be the cam as 820 says, but these engines are cammed for low/mid range torque for use with an auto gearbox, not high end power. The tapping noise, if it does it when warm, on a light or trailing throttle could be little end knock, they don't do it when cold.

Colin

Slow typing again.

[ChasR]

I have heard from a few people that the RV8s can potentially not rev all that well, but OTOH many do seem to be fine.

On my Stag which had near enough the same problem it was down to two things:

-curse word coil (even thought it was new!)
-curse word plugs.

With the first is it a genuine Ford Coilpack(s) or aftermarket? On my sister's old Mondeo 2.0 only genuine ones would be misfire/backfire free.

As for the Plugs that took alot of head scratching. Whilst my Stag stopped misfiring under load badly with the old Bosch Red coil reinstated it was still intermittent. Upon changing from NGK BP5Es to BP5ESs the difference was almost night and day! It idled smoother, pulled smoother and harder and with no sign of a misfire. As a matter of fact it ran very well after that right until the day I sold it .

To this day I do not know why but I can only put it down to one thing. The very first time I was tight and bought the NGKs from the cheapest supplier ; 4 of them came from Halfords in the plastic packaging where they were made in France, the other lot came from Japan, the ESs where all from Japan. I do now wonder whether the EU made items are inferior or low rent, thus cheaper ; it is no the first time I have seen this with 'OE' brands either. It is one of the very very few times NGKs have caused me issues TBH, and IMO it is a supplier/manufacturer issue. I guess cheaper is not always better .

[sowen]

May 3, 2015 18:30:13 GMT colnerov said:

Hi, it could be the cam as 820 says, but these engines are cammed for low/mid range torque for use with an auto gearbox, not high end power. The tapping noise, if it does it when warm, on a light or trailing throttle could be little end knock, they don't do it when cold.

Colin

Slow typing again.

My old Land Rover was a rocket compared to this one I've got now, same compression etc, and so far I've had the exact same carbs and Megajolt system fitted and it just isn't the same. It really does feel slow, I can chirp the tyres in first then it bogs as the revs climb past 4000. The tapping only started after I filled it with cheap 10W40 to flush it out and hasn't gone away since (it's got Millers 20W50 in it now). I'm going to guess the little ends will be quite terminal

May 3, 2015 18:41:48 GMT ChasR said:

I have heard from a few people that the RV8s can potentially not rev all that well, but OTOH many do seem to be fine.

On my Stag which had near enough the same problem it was down to two things:

-curse word coil (even thought it was new!)
-curse word plugs.

With the first is it a genuine Ford Coilpack(s) or aftermarket? On my sister's old Mondeo 2.0 only genuine ones would be misfire/backfire free.

As for the Plugs that took alot of head scratching. Whilst my Stag stopped misfiring under load badly with the old Bosch Red coil reinstated it was still intermittent. Upon changing from NGK BP5Es to BP5ESs the difference was almost night and day! It idled smoother, pulled smoother and harder and with no sign of a misfire. As a matter of fact it ran very well after that right until the day I sold it .

To this day I do not know why but I can only put it down to one thing. The very first time I was tight and bought the NGKs from the cheapest supplier ; 4 of them came from Halfords in the plastic packaging where they were made in France, the other lot came from Japan, the ESs where all from Japan. I do now wonder whether the EU made items are inferior or low rent, thus cheaper ; it is no the first time I have seen this with 'OE' brands either. It is one of the very very few times NGKs have caused me issues TBH, and IMO it is a supplier/manufacturer issue. I guess cheaper is not always better .

The thing is it had the exact same symptoms with the old Lucas electronic distibutor, rock hard leads and Esso spark plugs (I never knew there was such a thing as Esso branded plugs?), now it's running Ford coilpacks on EDIS8, new leads and NGK plugs with no difference. I've even replaced the alternator as that was only putting out 13 volts in hope that it couldn't supply enough current to fire all the plugs up the rev range?

I took my dad out earlier today for a run and the first thing he said was it sounded exactly like the first Mini 1000 engine he built, as the revs went up it made an almighty banging noise which turned out to be the valve springs not holding the valves closed. Replaced them and the fault went away.

This video kinda sounds the same at 0:22 as the engine goes over 6000rpm

[v8ian]

One thing that my RV8 would suffer with was a misfire, the main cause were the plugs, NGKs were the only ones that seemed to work, BP6ES, BP5ES would never stay clean,
The plug gap was critical, try closing your gap slightly, and the other real problem with RV8s were if you wet the plugs, take them out and throw them away, they will never run without problems, sound extreme, but that was the only way to get it to run.....and others who know will agree.....

[sowen]

May 3, 2015 20:35:35 GMT v8ian said:

One thing that my RV8 would suffer with was a misfire, the main cause were the plugs, NGKs were the only ones that seemed to work, BP6ES, BP5ES would never stay clean,
The plug gap was critical, try closing your gap slightly, and the other real problem with RV8s were if you wet the plugs, take them out and throw them away, they will never run without problems, sound extreme, but that was the only way to get it to run.....and others who know will agree.....

I've got NGK's in it and they're nice and clean. The misfire sounds like an almighty banging under the bonnet, it's very audible, not like any ignition related misfire I've ever heard or experienced before?

Thinking and talking with my dad about it we're going to pull the engine and strip it down, if there's the potential of one of the rods being at fault I don't want to experience any more rods trying to smash their way out the side of the alloy block, been there before. I need to sort out all the oil leaks and replace the gearbox so it makes sense to go all the way as soon as possible before the summer is over again.

[mdh]

Are you running resistor plugs?

[820]

If it is a clear misfire as you describe and you can hear it popping back/banging it could be running lean under load at high rpm, but if it did this with all the previous carbs and injection systems that seems unlikely unless it is fuel pressure related.

Engine knocking from small ends or pistons is bad but shouldnt cause a misfire or lack of power, I don't know that pulling the engine apart will help find this problem either unless the cam timing is wrong, might be worth solving this with the current engine rather than a freshly rebuilt one. Unless of course it is so bad it is likely to fail.

Considering the mileage it would be well worth doing a cylinder leakage test.

[v8ian]

Banging normally says lean out or the timing, as an experiment, take 5 degs out of the timing and see if the problem goes away, another thing that can contribute to misfires is the wonderful piece of plastic in the front of the engine called the timing gear, they wear very badly,

[sowen]

May 3, 2015 22:31:00 GMT 820 said:

If it is a clear misfire as you describe and you can hear it popping back/banging it could be running lean under load at high rpm, but if it did this with all the previous carbs and injection systems that seems unlikely unless it is fuel pressure related.

Engine knocking from small ends or pistons is bad but shouldnt cause a misfire or lack of power, I don't know that pulling the engine apart will help find this problem either unless the cam timing is wrong, might be worth solving this with the current engine rather than a freshly rebuilt one. Unless of course it is so bad it is likely to fail.

Considering the mileage it would be well worth doing a cylinder leakage test.

I've got an afr gauge and have been monitoring the condition of the plugs, everything I've tried has made completely no difference to the banging noise so far, comes in pretty much dead on 4000rpm. Fuel system is entirely different from when I started, there is not one piece of the old cab installation left.

My first Rover V8 developed a light tapping very similar, that ended with two rods sitting in the sump one day, I need the car to be reliable and my confidence is dropping fast with this one.

May 4, 2015 6:32:05 GMT v8ian said:

Banging normally says lean out or the timing, as an experiment, take 5 degs out of the timing and see if the problem goes away, another thing that can contribute to misfires is the wonderful piece of plastic in the front of the engine called the timing gear, they wear very badly,

I've been messing with the ignition timing on the Megasquirt, retarding and advancing it over 10 degrees either way, so far it's made very little difference tot he overall running of the engine, which is why I think there's something wrong deeper down.

I have just briefly wondered if the cam timing has slipped? The engine has so much sludged oil on the front cover that I don't think it's been off in a while, if ever, so would doubt a previous owner of garage has been in there and messed it up. Even so it would be easier to pull the engine and sit it on the floor to check instead of struggling to do it in the car.

[v8ian]

the map you are using does level the timing off at about 32degs?, have a word with Coops on the V8Forum he might have an old map before he supercharged his Capri,

[sowen]

May 4, 2015 7:11:49 GMT v8ian said:

the map you are using does level the timing off at about 32degs?, have a word with Coops on the V8Forum he might have an old map before he supercharged his Capri,

I ran it briefly with no advance, using the fixed timing of the EDIS module and the fault was still there.

[ChasR]

I still say the plugs . Where did you get the plugs from, where are they made, what model are they and how are they gapped? As said, my Stag was on NGKs, yet the problem went away with another set of NGKs.

I wouldn't normally question anything of yours but the issue does sound eerily similar to mine. Bar the banging the car would misfire at the RPM level as if it were a rev limiter short of gently revving up

If you listen to this poor video you can just about hear the misfire on the old NGKs and the banging that you could hear from the exhaust popping (and possibly the engine) just before I change gear at 1:51. Mine also sounded near enough the same as the car in your video.

[sowen]

Just watched you're video Chas, that's a light popping, nothing like what I'm getting. The old plugs were Esso (), I'll get the numbers off the new plugs in a little while when I return to the garage. I've never experienced an issue like it that cannot be affected by changes in the fuel and ignition system, almost sounds like it's blowing back through the intake?

Status update is the engine is mostly stripped down, timing cover off and the gears appear to be timed correctly, camshaft lobes are all present, and one head off, one to go. I want to check all the valve seats and spring free lengths next. I have a spare known good cam and lifters that I could swap in if I don't find anything conclusive with the original setup.

[deaddave72]

as everyone else here says, plugs. On all the rangies i worked on , i found champion plugs worked best and NGK were somewhat unreliable. As i have had an annoying misfire on my Carlton GSi recently , i traced it to the plug leads failing, that was also on the higher rev range as well. May be worth a new set if you don't know how old they are.

My 5 pennies worth

[sowen]

May 5, 2015 18:27:33 GMT deaddave72 said:

as everyone else here says, plugs. On all the rangies i worked on , i found champion plugs worked best and NGK were somewhat unreliable. As i have had an annoying misfire on my Carlton GSi recently , i traced it to the plug leads failing, that was also on the higher rev range as well. May be worth a new set if you don't know how old they are.

My 5 pennies worth

They're new plugs to replace the old ones with no change in the running issues. Leads are new on coilpacks replacing the entire distributor system, even tried another set of leads that were on my old V8 with the coilpacks...

There is no part of the fuel and ignition system that hasn't been replaced since I first got the car and found it had a problem in the upper rev range. I think the term misfire maybe a bit misleading, it's hard to describe but it's loud coming from the front, louder than a V8 with straight through exhaust being worked hard in a car with no carpets and little insulation

[820]

Is it consistent, as in will it do it every time at the same rpm? hot and cold? any gear?
I would still recommend a cylinder leakage test, much more info than a compression test.
Couple of things to look at if you are sure it is not fuel injection or ignition related.
Blocked exhaust
Fuel pick up in tank
Its tough to diagnose without experiencing the fault

[820]

Just realised it is in bits, cylinder leakage test no good now.
Measure cam lobes is best, and might as well fit new valve springs if they are original.
Hope you find it, good luck.