Car violently jolts when not under load

[ThreePointFive]

I have been battling with an issue that I thought was engine tune related but it's getting worse/more violent and it's definitely transmission based.

The car is pitching violently back and forth with accompanying clonks from underneath. It starts small and gets into a cycle where it's like the pitching sets off more pitching until you are being thrown around.

It will happen when rolling on flat ground and keeping a steady speed with the engine doing minimal work. Moving in traffic or rolling at 30 for example.

It's does not happen when accelerating or off the throttle and decelerating, or going up/down hill while maintaining a steady speed. It will not happen with the transmission under load of any sort.

It gets less pronounced at higher speed I assume because (being a brick) you're always under load of one sort or another.

Depressing the clutch will end the pitching immediately. Re-engaging the clutch in the same gear and no other changes, it will restart. Changing gear up or down can help but it's not a solution and it doesn't always make a difference (again, if the loading doesn't change).

It can happen in any gear, changes are typical LT85-buttery smooth and there's no undue noises from the box in any other operation.

The revs rise and fall when this happens but I think that's a symptom of the constant acceleration/deceleration of the jerking, not a cause. The engine is not stalling or running rough and would go straight to constant idle if the clutch is depressed.

My thought was layshaft/mainshaft wear and the play is hammering itself to death, but the shafts were replaced 500 miles ago.

I have another idea but I want to see what answers are and not influence thinking.

I can take a video if it'll help.

Any ideas? 

[western]

What state are the engine & gearbox mount rubbers in, they may have split or disbonded from their metal fixing plates

[reb78]

Handbrake?? It can cause a judder if poorly adjusted/binding but not as bad as you describe I would think. Simple check though just in case. 

[TSD]

I know you don't think it's engine tune related, but this sounds exactly like a 'tip-in' problem to me - in a carb'ed engine, where the fuelling moves from the idle circuit to the main throttle, and the two aren't well matched. It can happen with efi too, and with diesels. (My 2.8TGV can shunt horribly at light throttle at times, and while I've never spent much effort trying to cure it, mostly I just drive around it.)

As the conditions change as the engines comes off of 'no load', fuelling changes, timing changes, the engine moves on the mounts etc. Those changes can cause the engine to go back to 'no-load', and the cycle repeats. Changes aren't instant, and as backlash  in the drivetrain increases, the problem usually gets worse.

It can be as simple as a badly adjusted idle switch on an efi motor. Early megasquirters often put a small plastic fuel filter into the map sensor pipe to act as a damper. I've often thought of trying the same on the boost line for my TGV, but it's only been a dozen years or so, I haven't got around to it yet.

Edited December 18, 2022 by TSD
Hit post too soon.

[smallfry]

What sort of RPM is the engine turning when this happens ? If it is getting down to 12 - 1300 RPM it will be doing exactly what TSD has suggested.

My little diesel Combo van will do this, when I get down to about 18MPH in third and fourth, coasting either on the flat or slightly downhill, and what make it even worse is that the jerking makes your foot move slightly on the throttle pedal. It is overgeared though, as I changed the gearbox and diff. Did not do this before. 

For me, the solution is to change down a gear. This is another reason I love Autos !

[ThreePointFive]

Interesting thoughts, thanks for the responses.

I'll need to look at the exact RPMs but they've got to be upwards of 1200, it doesn't seem set to a specific engine speed but I will test that some more. I have set my idle high as it stops the cold start stalling issues I have in cold weather which might be changing when this kicks in. Normally I change up a gear as my solution, it drops the revs lower of course but it seems to flatten it out.

I had my money on mounts after some Googling, so I've ordered Glencoyne ones. I would be surprised if they've failed in 1000 miles, but they will be quite old by now - not that it's much of an excuse. It is the banging noises that accompany the motion that still make me think there is a huge amount of slack in the drivetrain. I am worried it's not going to be cheap.

I'll see if I can play with the tune a bit, and if there's any opportunity to get the mounts changed to at least rule them out.

[TSD]

23 minutes ago, ThreePointFive said:

I'll see if I can play with the tune a bit, and if there's any opportunity to get the mounts changed to at least rule them out.

Is this issue on a vehicle with Megasquirt? (I'm guessing based on username and your previous MS related posts) If so, you would likely be able to see it easily in a datalog, either the fuelling or the timing values will be jumping around massively if it's a tuning issue, and you should be able to work out what stimulus causes it.

Also, if it is 'squirted, and it's always had the problem to some extent, taking the MAP sensor feed from the wrong place can cause it, depending on manifold / throttle body design.

Shunting caused by bad engine or gearbox mounts is usually noticed first as a juddery clutch, in my experience. Worth checking you haven't a very poorly UJ though, if you haven't already. I had one that gave no symptoms at all, except when going from first to reverse sharply when parking. I heard it clearly then, and I was a bit shocked how bad it was when I dropped the prop.

 

 

 

[Snagger]

I didn’t get vehicle pitching but the noises and vibration when coasting did occur on mine when I had prop UJ issues.  They were wearing out quickly because of the rear diff being inclined and so the pinion axis and gear box axis being misaligned.  I’d look there first.

[landroversforever]

9 hours ago, ThreePointFive said:

I have set my idle high as it stops the cold start stalling issues I have in cold weather which might be changing when this kicks in.

Have you tuned the warm-up enrichment? It's not a V8, but the only car I've squirted, and it starts and idles beautifully after a few tweaks to the WUE.

[Bowie69]

It's tune related,  I guarantee it.

[monkie]

19 hours ago, western said:

What state are the engine & gearbox mount rubbers in, they may have split or disbonded from their metal fixing plates

I second this. When I put a new set of quality mounts (engine and gearbox) on my 110 the difference was night and day. 

[landroversforever]

11 minutes ago, monkie said:

I second this. When I put a new set of quality mounts (engine and gearbox) on my 110 the difference was night and day. 

Mounts would make a difference when coming on and off load though, not at a constant throttle. I’m with Bowie, I reckon it’s tuning related. Especially if the idle is set high. 

[monkie]

Agreed, but old/bad mounts would make the situation feel worse I think

[FridgeFreezer]

4 hours ago, Bowie69 said:

It's tune related,  I guarantee it.

Same here - took me a while to get the 109 to drive smoothly when trickling along in low-range as the engine has so much more "leverage" to cause this shunting effect when you're in low range.

The ambulance still does it under light throttle loads and that's on my list of jobs to do once I get the fuelling into a sensible ballpark.

All that said - I'm awful at engine tuning so would defer to Bowie and TSD's advice on causes & solutions, and indeed I'll be following and taking notes myself

[Bowie69]

2 minutes ago, FridgeFreezer said:

Same here - took me a while to get the 109 to drive smoothly when trickling along in low-range as the engine has so much more "leverage" to cause this shunting effect when you're in low range.

It's an unexpected part of the map, very low MAP, and low revs, just above idle, so never gets touched by auto tuning really, but vital to low speed crawling ability. Fundamentally you need it smooth, not sharp changes and watch why it is hunting around while driving and then fix the numbers to something better -can be timing or fuel.

Overrun fuel cut off can also cause the same issue, if you set it too low.

[ThreePointFive]

5 hours ago, Bowie69 said:

It's tune related,  I guarantee it.

But I haven't brought it to you for tuning until I get this pitching issue sorted..... 

 

I'll look at the tune. I think it's a mixture of worn mounts, a lot of play in the driveline and this map issue all combined.

I'm not ruling out UJs though, I'm not messing with those going wrong.

[TSD]

52 minutes ago, Bowie69 said:

Overrun fuel cut off can also cause the same issue, if you set it too low.

This, and potentially overrun timing if implemented, are the most likely causes in my experience. Followed closely by users who think auto-tune is a good idea Only joking, though fridgefreezer will confirm I'm a confirmed high tech luddite.

I haven't messed with megasquirt for many years, but at one point I ran experimental EDIS on a V6 specifically to try to put in some of the engine braking that Ford left out of the Capri. Similar to the RV8, the Essex engine had a terrible combustion chamber design and needed loads of advance to gets best performance under acceleration. Trying to get more engine braking off-throttle might mean going from 30+ degrees advance to 10 degrees retarded, which EDIS could do instantly, accompanied by lots of popping in the exhaust and an almost undetectable amount of extra engine braking. Oh well, it wouldnt be an experiment if it always worked out, and it was a lot of fun

[Bowie69]

1 hour ago, ThreePointFive said:

But I haven't brought it to you for tuning until I get this pitching issue sorted..... 

I've tried a few times but I think you re-worded this sentence mid-stream....

If you drive it and get it shunting, if you see the marker on the VE/timing map running round in circles you have your problem.

[Junglie]

I had this on an Alfa Romeo a few years ago - it was the MAF sensor that caused the trouble but it mullered the engine mounts as well...

Do we even have MAF sensors? If we do they're always worth a suspicious look. If we don't then treat this as a mildly interesting but irrelevant anecdote.

[ThreePointFive]

45 minutes ago, Bowie69 said:

I've tried a few times but I think you re-worded this sentence mid-stream....

If you drive it and get it shunting, if you see the marker on the VE/timing map running round in circles you have your problem.

I didn't edit that part, not sure what you saw! What I meant was, I didn't want to bring it up and waste your time if it had obvious non-tune issues I needed to sort first but it seems it's all the same.

I'll take it for a drive and try out the data logging. 

Junglie, yes the Megasquirt kit is MAF based so this is also adding into the engine side being the cause.

[Bowie69]

Just now, ThreePointFive said:

Megasquirt kit is MAF based

Are you using a MAF..............?

 

1 minute ago, ThreePointFive said:

it seems it's all the same.

Yep, I think it is.....

[ThreePointFive]

16 hours ago, TSD said:

Is this issue on a vehicle with Megasquirt? (I'm guessing based on username and your previous MS related posts) If so, you would likely be able to see it easily in a datalog, either the fuelling or the timing values will be jumping around massively if it's a tuning issue, and you should be able to work out what stimulus causes it.

Also, if it is 'squirted, and it's always had the problem to some extent, taking the MAP sensor feed from the wrong place can cause it, depending on manifold / throttle body design.

Shunting caused by bad engine or gearbox mounts is usually noticed first as a juddery clutch, in my experience. Worth checking you haven't a very poorly UJ though, if you haven't already. I had one that gave no symptoms at all, except when going from first to reverse sharply when parking. I heard it clearly then, and I was a bit shocked how bad it was when I dropped the prop.

 

 

 

Sorry, meant to confirm that yes I am operating a MS system on a 4.0 V8.

 

Annoyingly I've got it to run the best it has been in a long time, but the kangarooing did start around the last few tune iterations. I just put it down to coincidence as I don't drive it very much unless it's to tune it or drive off road where I might have ruined the underpinnings.

[FridgeFreezer]

4 minutes ago, ThreePointFive said:

Junglie, yes the Megasquirt kit is MAF based so this is also adding into the engine side being the cause.

Um, it's not MAF based, there's no MAF, it uses MAP

MAF = Mass Air Flow, flapper or hotwire sensor in the intake pipe measuring the volume of air going in

MAP = Manifold Absolute Pressure - measure the vacuum, calculate the amount of air going in from that.

@Bowie69 I'm pretty sure Sam doesn't have a MAF as I've looked at his ECU before to rule that out.

Sam - although this may be exacerbated by drivetrain slack or wobbly mounts, it's almost certainly the tune that's causing it, and as Bowie and TSD say, it's a part of the map not really visited "in anger".

[ThreePointFive]

4 minutes ago, Bowie69 said:

Are you using a MAF..............?

Isn't that the sensor hanging out on the intake pipe?

I give up. I shouldn't be allowed near cars.

[Bowie69]

Engine bay photo required.... 

If its a Nige kit it's more likely an IAT in a holder/brackety adapter ring....