Hesitation very annoying

[petezipp]

Help i'm confused Just turned 90k miles.
had this problem for some time now but can't decide if its Engine or transmission.
When driving from cold some hesitation at low revs ( sort of jerky) and on overrun, as it warms it reduces but always there, not serious but it spoils the feel of the car.
Can't feel it through the steering so I guess it's not wheel balance, but it feels like something is out of balance.
It is a fore/Aft movement not sideways, if that's a help!
3yrs ago i had the dreaded jack shaft bearing bolts come loose so I locktighted them , Could i have bent it ? would the bearing be noisey if turned slowly or do I have to attach myself to the underside of the car whilst travelling along ( not quite that mad -yet)
Replaced one Drive shaft boot last year but no change, and they seem ok.
Just can't see how to narrow down the fault, but I thinks it is slowly getting worse.
peter

[Enright]

Try coil packs first. And plugs. Even if it turns out not to be those you'll need some sooner or later, so they won't be wasted.

[rip]

Maybe Elanscan could help here? Do have access to it?
If so, a controlled run which someone else could replicate in their Elan to provide a comparison could help (not just an energetic blat around some country road).
A good scan to start with is to start logging before you start the car from cold & just leave it idling until it gets warm.

[Peter D]

Let's hope it's not what I experienced...
After hours and hours of troubleshooting with Elanscan, replacing sensors, coils, plugs, ... we checked compression...
Compression leak test result:
Cilinder 1 had a 20% pressure loss towards exhaust, cilnder 2 blew into cilnder 3 (35% pressurre loss).
When the head gasket was changed, nothing visible on the old gasket nor the cilinder head.
Even loss in cilinder 1 couldn't be confirmed by the rebuilder who took care of the revision, and only 0.06mm or so was skimmed off just to have the surfce 100% clean.
After rebuild, compression as new, and it now runs as never before.
(Piper downpipe and Everest chip on the S2)

So when you change plugs, perform a compression test.
If only to reassure yourself of your engines mechanical health.

[Brit-Car-Nut]

A compression test is only part of the diagnosis process when cylinders seem to be unbalanced.

You should also perform a leak down test which will help to identify where the compression is going. It could be a cracked or burned valve or valve seat, worn piston rings or piston lands or blow-out into the oil or water jacket.

Usually the tools to perform these tests are available for rent so you don't have to invest in them. The leak down test tool also works well to identify intake manifold and vacuum leaks by creating a fixed air flow that can be introduced into the intake system.

I would suggest first checking the spark plugs for correct application then make sure the air gap at the electrode is correct. Watching the engine run in the dark with the plug cover off might reveal if the plug wires are leaking (it will look like a light show around the wires as they allow the high voltage to ground out and also contaminate the wires to the other plugs).

Has the fuel filter been replaced recently? Have you checked to make sure water hasn't gotten into the petrol due to a bad filler cap or clogged drain tube?

Have you checked the base timing? When was the last time the cam belt was checked or replaced?

There are quite a few issues that could cause a hesitation but ignition is often the best place to look. Use an ElanScan if you can find one and as suggested, start logging before you even start the car. Warm it up and drive sensibly to get the best average readings. At the very end, you could try to induce the hesitation so it can be identified as a variance from the average.

[Peter D]

By the way, this all happened shortly after installing the SAMCO induction hose kit.
It's described in another topic here on LEC.
If you missed it, SAMCO hose delaminated, damaged the compression side of the turbo, intercooler full of orange silicone, ...
So please be warned, do not fit his until SAMCO brings a solution.

Now running with the stock intake hose, using the opportunity to install the piper downpipe wilst engine rebuild.
After the O.K., I installed the Everest, having it operational only a couple months later.
Had some issues getting my car running on the Everest chip, though Alan supported me in sending replacement.

Sorry, going off-topic here...