Knocking

[67_gt6]

Hi

 

My engine runs like a sewing machine ordinarily. Mixture and timing were set by eye/ear/swing method etc and have caused no issues in 9000 odd hard driven miles. The distributor is a Martin Jay job with about 9k miles on it. Ignition is Pertronix Ignitor. Leads are Magnecor. Coil is mounted on block. Filters are K&N. It rarely runs on and redlines with gusto. There are a lot of cobbles in Edinburgh so the car gets flung about a bit, but anything that was likely to drop off has already done so

 

A few days ago I developed an intermittent misfire. It seemed to happen mostly at low RPM and I could accelerate through it, like you might if a plug was getting a little fouled and then clearing. Then I happened to notice that one of the carb dampers had become unscrewed, so I thought my low RPM misfire had been caused by that, but the misfire returned.

 

Then I noticed the idle RPM was about 100 higher than normal, at first I had put this down to recent slightly colder temperatures but then I noticed a split in the hose from inlet manifold to PCV valve so there was a leak. I bodged a temporary fix with a lot of insulating tape and idle returned to normal, am awaiting replacement.

 

However, then the misfire grew into bad pinking that occurs after 2000 RPM, most obviously in 3rd and 4th on the flat at part throttle. I limped home and checked the plugs. They all looked absolutely normal apart from one which had a lot of ash on it. I use Castrol Valvemaster Plus and had recently started a new bottle.....just can't be sure that the problems started with the new bottle and needless to say I've got a full tank of fuel at the moment. I also had a look at the distributor advance weights and springs and everything seems to be normal.

 

I replaced the plugs and took it for a run last night, it seemed ok and the plugs looked great afterwards. But then on the way into work this a.m after about 10 mins the knocking started again, bad enough that the engine feels like it won't go over 2000 RPM without lunching itself. As a last throw of the dice I swapped the plug leads and for a while it seemed ok, but just not "right" and still a bit reluctant to rev. I stopped and retarded the vernier about a section and a half and completed my journey as it's a fast B road and I was worried about not hearing knocking.

 

I guess that all things being equal it must be the fuel as that is the only thing that has changed? And, as it's a knock that is occuring now that would tend to point more to fuel than something going wrong with electrics? My idle is absolutely smooth and normal and the distributor is firmly fixed and hasn't moved. It seems to rev normally when not under load.

 

Will be doing a compression test tonight to make sure I haven't suddenly developed high compression in one or more cylinders, and I'll start topping off the fuel frequently with 97 RON. I guess I could have carbon build up.

 

Any other theories welcome, I appreciate there are quite a few likely red herrings in the background to this story!

 

Andy

Edited October 15, 2008 by 67_gt6

[Dazzer]

Hi Andy

 

Have you actually checked the timing with a strobe, at tickover, vac advance removed?

 

As what you describe sounds a little too severe for incorrect fuel grade.

 

I run a TR4 on Shell Vpower and I have the timing slightly advanced up from the 4 btdc (6 btdc) If I cant get Vpower I have used standard 95 unleaded and I get a little pinking, under light throttle opening, or under heavy load uphill, but nothing as severe as you describe.

 

If the timing is out and has moved noticeably from where it should be I have seen the distributor drive dog 'pin' shear and if the drive is a tight fit it will just slip a little. What usually happens is that the timing is a mile out when you check it, you can reset it but a few hundred miles later it slips again.

 

This is a rare fault though and you'd have to take the distributor out of the car and place it in a vice to check.

 

If the timing spot on however it must be the fuel, drain it and fill up with some super unleaded (VPower preferably)

 

Hope this helps, look forward to hearing the outcome

 

Cheers

 

Darren

[Nick Webster]

This could be serious. Reading your story. I'm a bit confused between pinking and knocking. Pinking is pre-ignition, but where is the knock coming from? Knocking is usually some mechanical part in distress.

 

What kind of ash was on the spark plug? Metallic grey flakes? Worst case scenario - If the engine has been running very lean due to the split hose and also has a tendency to pink, there is a chance you have started to burn out a valve. Also possible is a damaged a piston crown, which might be picking up on the liner walls and the engine is seizing slightly as it warms up. Unfortunately it is difficult to check all this without taking the head off.

 

Nick Webster

[67_gt6]

Hi guys

 

To clarify - pinking and knocking are one and the same in my post.

 

I've experienced splits in these hoses before, it's not a great design as they get very hot. When it happened on my GT6 I'd get a slight increase in idle and maybe a little bit of a lean stumble at low revs.

 

Anyway, new hoses arrived from Revington and I fitted them at lunch. returned dizzy vernier to approximately where it was originally. had a look at the plugs and they looked like they'd been seeing a leaner mixture than they did last night when I thought I'd fixed it.

 

Took the car out for 5 miles or so and it appears to be fixed. I think my temporary bodge on the hose was just not enough to prevent the air leak. Possibly wide open throttle was removing the vacuum that was keeping the leak appriximately closed and thus I was getting a very lean mixture?

 

Anyway, proof will be in the pudding as I've got a 16 mile drive home tonight.

 

Thanks

 

Andy

[BrianC]

...I think my temporary bodge on the hose was just not enough to prevent the air leak. Possibly wide open throttle was removing the vacuum that was keeping the leak appriximately closed and thus I was getting a very lean mixture?

If the hose was old enough to split, then it could also have been collapsing internally. Not sure what effect this would have, but I doubt it would be beneficial.

[67_gt6]

If the hose was old enough to split, then it could also have been collapsing internally. Not sure what effect this would have, but I doubt it would be beneficial.

 

That's true Brian. Age isn't the issue really, they were replaced about 3k miles ago. They don't seem to last very long, it is usually the one that sits nearest the manifolds that goes first.

 

andy

[ChrisR-4A]

Hi Andy, have you tried replacing the ignition module with some standard points, I have heard of two of these electronic units failing recently, but only partially and still allowing the engine to run but with much reduced power.

 

Chris

[67_gt6]

Hi Andy, have you tried replacing the ignition module with some standard points, I have heard of two of these electronic units failing recently, but only partially and still allowing the engine to run but with much reduced power.

 

Chris

 

Hi Chris

 

The problem was definitely the air leak as it was instantly back to normal once the hose was replaced. I agree though, I was beginning to suspect something on the dizzy side, but the problem seemed to be consistently centred on a certain range of RPM which made me suspect mechanical distn. problems rather than the electrical module.

 

cheers

 

Andy

Edited October 17, 2008 by 67_gt6