diagnosing poor engine performance-- sound familiar????

Big lumps of metals and spanners. Including servicing and fluids.

Moderators: User administrators, Moderators

diagnosing poor engine performance-- sound familiar????

Postby TheProf » 08 Apr 2018, 21:21

[apologies for the essay!!]
Just in case this helps any members out with any ideas, or just to provide hope for those trying to diagnose their
low power, poor fuel economy, poor running, occasional backfire, occasional cut out ... engine problems, the following
is my recent story ...

The engine is a stock 2.1dj with standard fuel injection setup in a standard (non-camperised) 1990 syncro caravelle.

The symptoms were:
gradually over a period of a few months the driving experience went from smooth and powerful running at all speeds and returning over 20 mpg on most runs, to very poor fuel consumption (13mpg), rough starting and very poor about town driving experience not improved once warmed up, (lack of power - hesitation/dying when gently depressing accelerator, but not quite stalling, especially on trying slow speed tight turns, or trying to accelerate under load suddenly; occasional backfire when easing off high revs in low gear).
But it would drive ok at motorway speeds on the flat (when it got to speed) -- smooth-ish running-- but then coming to a stop
at a roundabout the engine would occasionally just cut out, but would also start immediately again.

So before reading on ahead, if i had posted this description on the forum, what would your suggestion have been for a possible fault?

A check of the spark plugs showed black and sooty deposits on all plugs -- using standard single electrode plug, gapped to 0.7mm.

So the dying and loss of power when depressing accelerator was due to overfuelling (and explained the 13mpg)... .
the decent motorway speeds and handling probably ruling out fuel pump/filter,injectors..

My first thought was the temp2 sensor as many of you would say, but a) this had been replaced not long ago, and b) checking the temp2 wiring and sensor resistance at the ECU plug was in the expected range according to the bentley manual..
Whilst I was at the ECU plug I also checked the resistance of the injectors (one of them was out of spec - just - but that would not
have created the problems). I also checked the Hall sender and sensor wiring at the ECU plug according to bentley which was also out
a bit, but the symptoms were the same hot or cold anyway, possibly indicating it was not a fault with the Hall sender.
I also checked the input air temp sensor resistance at the AFM (fine), the AFM throttle position switch
(it checked out ok - nice click - switching open/close verified with a voltmeter at both extremes), checked Idle Stabilisation Valve
(buzzing just fine), and swapped out the ISCU (control relay box behind the offside light cluster) with a spare
(but no change in symptoms). I also verified that the crankcase breather pipe into the ISV and from ISV into the throttle body air
intake were clear.

By this time I had swapped out plugs, leads, cap, rotor arm as I had them ready for a service anyway.

So it *must* be an air leak I thought - which would explain incomplete ignition, stalling at roundabouts, rough running, lack of power,
but wouldnt this make it burn lean not rich on a dj ?? Anyway, progressively and systematically worked though the air intake system now --
I remade connections between the air filter housing to the throttle body, from the throttle body to the plenum chamber
(yes, a very thin rubber seal having seen better days which I was hopeful was the cause - replaced it - but with no or
minimal effect).
I also checked the vacuum pipe from the throttle body to the advance vacuum unit, and the hidden pipe from the plenum to the
other vacuum outlet on the dizzy vac unit (the dj engine uses a dual port vacuum unit, so has more pipes to make good).
(btw -- thanks to aidan for reminding me of this - and also checking that the pipe into the plenum hadnt become blocked) ..
I also visibly checked the rubber collars for the plenum to manifold (and listening for whistles on opening up accelerator). . .
no obvious leaks, but a job in the near future. .

Also visibly checked fuel feed pipes fuel rails and pipe connectors for possible leaks or cracks for air leaks into the fuel,
but they have all been replaced in the engine bay with non braided hose and no cracking was evident, and hoses all pliable.

I removed the dizzy cap and sucking on the advance vac pipe had resistance and some small movement of the plate in the
dizzy could be seen.
This is what threw me for 2 days as i assumed this vacuum unit was fine although it was very very very corroded on the outside!!!

So if the signals going into the ecu were ok (and most of my tests seemed to indicate they must have been, although I could not check the
signals when the engine was running), and no air leaks then the ECU must be faulty, holding the injectors open too long. ??
I borrowed an ECU believed to be ok -- but problems persisted, So now very confused.. of course the borrowed ECU was not
guaranteed perfect as it was a spare, not from a running van -- so... I redid all previous tests. No improvement.
I remade all possible joins for air leaks again .. no improvement.
It was as if the timing was not right and the van was driving in a limp-home mode. But the timing had not been
touched since i bought the van a couple of years or so ago.. so having eliminated all of the cheap possibilities it was pointing towards either the ECU, the throttle body itself, or the distributor.

At this point, yes, I should have borrowed a known working distributor from a club member to rule out the dizzy. But it couldnt
be the hall sensor as the symptoms didnt fit. So by elimination and costs (working cheap to expensive!) since the vac unit
looked really corroded I decided to replace it anyway --
I bought a new unit from Baxters .. stripped the dizzy down, and replaced the vac unit. --( tip: if you do this job, invest
in some small strong circlip pliers !!! dont try and use two screwdrivers like me -- what took 3 hours should have been
done in less than an hour including cleaning, if the rusty screws had come undone!).

Anyway - refurbed distributor back in (btw --before you remove the distributor, line up the rotor arm in the dizzy and mark a couple of places between the dizzy body
sighting along the rotor arm so you dont upset the timing), start engine.....

-- It worked!!!! All fine! Back to normal.. Smooth start, smooth driving experience around town, power uphills again !!
so -- what must have been a small fault inside the rusty vacuum unit made for a very unsatisfactory driving experience that
took a total of more than about 5 days to sort out.. still -- i got to do several jobs in the engine bay i was intending to get round to at some point, ... and some others i didnt know about :-)

Anyway, for anyone having the "engine running rough, cutting out, smells of petrol..." problem, it just might be the temp2
sensor if you are lucky. But it could be any number of other things, as the forum experts will advise.....
so try and keep checks methodical, and you should be able to at least eliminate causes and eventually track down the

so.. how many guessed it was a failure in the vacuum unit?
1990 rhd syncro trakka 2.1mv petrol/lpg
1989 rhd syncro 'velle 2,1dj petrol
Posts: 35
Joined: 19 Sep 2008, 20:21
Location: malvern
80-90 Mem No: 9683

Return to Mechanical

Who is online

Users browsing this forum: No registered users and 47 guests