Rover V8 juddering on fast idle (1500-1700 RPM)

jonny m · Post by **jonny m** » Sun Jul 28, 2013 6:52 pm

This may have been happening for a while, I only spotted a few days ago, and car is still driving OK

Idles OK, but as I creep the revs up when parked, the engine judders / shakes / vibrates. I noticed it through feedback on the gear-stick when in neutral. The shudder seems to disappear once past the 1,500 - 1,700 range

I've used a spare spark plug to test each lead is getting a spark, and inspected all the silicon leads for damage, as my first hunch was a cylinder not getting a spark - all seems OK. Replaced the plugs with new, and moved the dizzy angle, and still juddering just at 1500-1700 rpm

(I do occasionally get a bit of a backfire when backing off at high revs, but put this down to a touch of over fueling and weak seal at the bottom of the exhaust manifold - not sure is relevant)

When I saw the shudder I thought it was the alternator, but had a digital measure on it and the water pump pulley. Pump was +/- round by 1mm, the alternator was perfectly round. When I fired the car up without the fan belt, the judder seemed much less noticable - which makes me wonder about the bottom end, crank etc...

It's a 3.5 block, and oil and water seem fine, ie not mixed

Any ideas or suggestions ???

Jon

ChrisJC · Post by **ChrisJC** » Sun Jul 28, 2013 8:45 pm

Does it do it under load?

Chris.

jonny m · Post by **jonny m** » Sun Jul 28, 2013 10:31 pm

Can really tell, it's in a Cobra kit car with rock hard suspension.... everything shakes when it gets going.

It isn't noticeable when moving as such, why, what you thinking?

DaveEFI · Post by **DaveEFI** » Mon Jul 29, 2013 8:14 am

First thing I'd do since it's free is measure the resistance of all the plug leads.
Difficult to give an exact figure as they will vary by length and maker. Something like 5000 ohms per foot. What you're looking for is one (or two) which are wildly out. Check the king lead too. Also look for signs of corrosion on their connectors.

ChrisJC · Post by **ChrisJC** » Mon Jul 29, 2013 5:04 pm

I just wondered if it was mechanical balance - it would do it when spinning down after a rev if it were mechanical, but if it got worse under load, it could be a misfire / mixture / ignition problem etc.

Chris.

jonny m · Post by **jonny m** » Mon Jul 29, 2013 10:04 pm

It's just as the revs when parked rises to 1500-1700 rpm, then is seems to lessen.

Once foot of the pedal and it drops back to idle, all is fine

I'll test leads, or swap for a spare set I have and see if there's a difference. My hunch / hope is a weak lead or shorting- TBA

mgbv8 · Post by **mgbv8** » Mon Jul 29, 2013 10:26 pm

From what you describe I dont think its anything to do with plugs or leads etc.

You dont notice it under load when driving.
The engine does not seem to misfire.
But when you ease the rpm up from idle you get a vibration from 1500-1700 at which point the vibration goes away?

Sounds like a balance issue to me. Not sure if you are manual or auto. But it sounds like fly wheel / clutch or flex plate balance issue.

Have you had the rotating assembly balanced?

A vibration noticed while on the way up in rpm past the point of vibration does not always show itself on the way down the rpm's.

We have the same problem on my Sons Escort V8. Mainly because we were in such a hurry to get it built that we didnt bother balancing the flex plate when we welded the lugs on for the torque converter. That hits a vibration point from idle at 2000 rpm as you gently creep the revs up. But it doesnt vibrate on the way down. We dont notice this problem when its under load while racing. But its deffo the flex plate out of balance.

jonny m · Post by **jonny m** » Sun Aug 18, 2013 9:04 pm

Right, new leads fitted... no change, still a shake at 1700rpm. So, in my mind, that's electrical ruled out.

I had the flywheel lightened in my engineering workshop, not too much, just a light skim.

So the balance theory is quite possible; it isn't noticeable when driving.

To be fair, it's only a light shake... should I worry, or leave and just drive and enjoy ?!

ChrisJC · Post by **ChrisJC** » Mon Aug 19, 2013 7:17 am

I would just put up with it until I had to remove the flywheel for another reason.

Thinking about it, Land Rover dealers could balance the engine in-vehicle. Maybe that could still be done?

Chris.

unstable load · Post by **unstable load** » Mon Aug 19, 2013 7:24 am

Sounds like a balance issue to me. Every rotating part that is even slightly out of balance has a point where the out of balance is able to be felt at it's worst. Yours seems to be at around 1500-1700 rpm.

The fact that "it goes away" over those revs merely means it has gone into a frequency range where it is less pronounced to your Mark 1 seat of the pants test method. As speed increases the perceived vibration goes down, because the seat of your bum or your hand is not that accurate a measuring tool.

You say you had the flywheel balanced so that should be your first point of investigation, merely because logic dictates that if something has changed, then the last thing that was worked on is the likely source of the change.
As much as it's a serious ball-ache, I'd recommend you drop the box and first remove the clutch for a good inspection. It could be the pressure plate has lost a spring or a bolt has come loose or is a different length to the others.
If that is all good, then remove the flywheel and send it to be dynamically balanced.

Have a look at the link below for some ideas of what could be the source of your woes.
http://www.vibratesoftware.com/html_hel ... engine.htm

jonny m · Post by **jonny m** » Sun Sep 08, 2013 8:23 am

It would help if the engine mount bolts were tightened... Dohh!

Plus, the water pump pulley is a tad out of alignment, drives fine, so not going to worry about it