Engine resonance.

[SteveJT]

I'm just wondering if anyone has had experience of a resonance that increases from about 50mph upwards, around 2500rpm onwards. From a previous owner's records it is suggested it comes from the area of the alternator. I'll be checking the belt tension and bearings presently. It did seem to reduce when I topped up the coolant header tank.
The engine has been uprated but I don't know the full details, it gives 211bhp at 6900rpm, the flywheel has been lightened and the carburettors (DHLA 45) re-jetted. It has a 4 branch exhaust into the rear custom made silencer. I imagine it is nearer to the HC spec, a rough compression check with my original (1978) gauge gave around 170psi.
I know this is a long shot, this seems to be my main concern at the moment.
I've made 3 trips to lotus (and back!!) for parts since purchasing last June, about a 380 mile round trip. It's a bit noisy but ok around 55 to 65 mph. Hopefully as time goes by the vibes and noise can be reduced, we keep on improving.

[Hawaiis0]

The Excels are prone to having the power steering pump bracket bolts shear or drop out.

Has the tensioner pulley been replaced?

Disconnect the alternator belt and see if the resonance goes away.

Custom pipes might be a consideration too.

[bash]

Resonance is usually exhaust related in most cars. Ive never had it in the excel with either the lotus engine or the v8, both of which use the standard system, I never had the 4 branch manifold on the lotus engine though. If it is the exhaust cruising at the speed at which its at its worst and then say dropping it to 4th and keeping the same cruising speed should affect the resonance. Im struggling to figure out why topping the water up should affect it. Vibrations though could be a number of things like engine and gearbox mounts and alot of us have a slight driveline vibration at a certain speed, although mine disappeared when I rebuilt the diff. What part of the country are you in, perhaps a local Excel owner could give you a comparison ride ?

Bash

[SteveJT]

Hello Bash and Hawaii.
Thank you for your help. (No power steering!!).
During the winter stoppage I did find both engine mount bolts slack, they're tight now although I will plan to replace the mounts next winter.
It's a good idea to remove the alternator belt and try a run, thanks.
The water reservoir was just about empty when I first checked, the level is about an inch from the top now and the car is now more steady around 92 degrees.
The exhaust is modified from original, I don't know when the resonance started but the comment about the resonance being near the alternator was made by a sportscar specialist.
In 2020 the ignition timing was adjusted during the rectification of a misfiring fault and a new TDC mark made on the flywheel. the carbs were also checked, float heights slightly adjusted and balanced, all done by the sportscar specialist. (I'll be checking this during the next winter SORNing).
The cam belt and tensioner were replaced in 2021 by the same company. They did give a pretty comprehensive account of what was done and further recommendations. The intention was to replace the alternator belt but that supplied to them was too short so the original was kept. Since then the car has covered 6500 miles approx.
It's an interesting problem, one that in time I hope can be solved. It does spoil the driving a bit, I'll update with any findings.
I like to run the car during the better weather and salt free roads, generally cruise around 55 to 65mph, never in a hurry to get where I'm going to!!
I'm in Sandwich, Kent. It's a bit wet now.

[Pete Boole]

Just noticed it's an Esprit we are talking about - hence no power steering, timing marks on the flywheel etc...

Pete

[rbgosling]

A couple of experiments you might want to try...

At a steady engine speed, does it seem to vibrate more if you open up the throttle rather than cruising with a fairly closed throttle? If so, it could be gas pulses exciting vibration in the exhaust system. If this is the cause I'm not sure there's much you can do about it - unless there is supposed to be a rigid mount between the downpipe and the sump or gearbox or something that is missing, I don't know what the exhaust layout is in an Esprit.

If the vibration is much the same whatever the throttle level, then it's more likely to be mechanical.

You could use the idle control to increase the revs to where you're getting resonance, then you can just watch the engine and see if you can see anything obviously vibrating. Maybe use the slo-mo video feature on your phone, but I don't know whether that slows it enough to be really helpful.

[SteveJT]

Thanks rbgosling, all worth trying, good idea on the exhaust. I will try some procedures with the speed and rpm. I've just noticed a comment on the rear offside wheel bearing feeling a bit rough. I didn't notice anything when I had it on my 2-post lift but I wasn't looking for anything like that.

Steve.

[John]

Hi Steve, I would also check that anything in the engine bay that could vibrate is secured properly. The Esprit has an engine cover doesn't it? If thats loose could it be vibrating at certain speeds?

Also check that everything else in the engine bay and boot is secure - heatshields, airbox, tools, lamp covers etc

Good luck

John

[SteveJT]

Thanks John.
Yes, I have emptied the boot area, didn't consider the engine cover but don't really want to run without it so will secure it a bit tighter. Have tried different rpm and speed combinations. Nothing stands out but it does occur at 70mph at 3500rpm in 5th, reduces in 4th same speed and also reducing speed and keeping same rpm in 4th. a really weird problem, one to be solved one day. The car is back in my garage now ready for lifting to check the rear wheel bearings with a bit more focus than when it was up last time.

[John]

Hi Steve, must be annoying, hope you can trace it. Is the resonance a rumbling, booming or rattling? As it's only noticeable at certain speed/rpm, it does sound like exhaust most likely, especially as the car has a modified system. I'd double check exhaust mountings and engine mountings, plus all exhaust joints in case any are loose. I'd check that the rubber pads in the engine mounts are not perished as well as making sure the bolts are tight. They're knackered on my Excel.

It could also be something vibrating inside the silencer but that wouldn't be easy to check.

Cheers

John

[bash]

I had a TVR vixen once, that had a bad resonance from the exhaust on the motorway, it went away at 90. Cameras on the motorway have spoilt that fix.

Bash

[AndrewWebber]

If you get nowhere and want more expert assistance, in my one time experience with a different issue, these guys are really good and may be worth a visit.

https://vibrationfree.co.uk/

Vibration Free near Bicester.

Andy

[SteveJT]

Thank you Andy. I'll keep them in mind, never against a visit to Bicester, near Jaguar. Had a few visits there too.

[SteveJT]

Many thanks to everyone for the suggestions, it's been a while from the last writing but progress has been made.
I'm coming to the conclusion that the resonance is a result of a number of areas that are vibrating at different frequencies that merge and then separate. Initially the periodic time was about 5 seconds and the amplitude / volume was noticeable around 50 - 70 mph.
I tried several things and some made a difference:
Topped up water reservoir, tightened both engine mounts (both long bolts slack), tuned carburettors. all made an improvement, especially with the amplitude and volume.
When retuning the carbs found the gearbox cover vibrating. With it removed found half the jack nuts slack and one missing. Tightened the nuts in place and tried a run without the cover, noticeable improvement. The periodic time had lengthened to around 10 seconds and the amplitude / volume noticeably reduced. The cover is showing signs of vibration around most of the fixing bolt holes.
We have a soft rubber strip now fitted under the cover where it sits on the body and larger washers for the bolts. All is tight.
Finally, I've been talking with an engineer at Opie oils who has been a real help. He has recommended a change of viscosity as the oil only heats to about 81-82 deg C into the cooler and 78-79 out. There's no thermostatic control valve, but I'm thinking about fitting one. I have yet to do the oil change but am hoping for the best.
Hope there aren't too many of you fallen asleep and that this may help a few.
Happy days, summer's coming!!