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Fuelling issue - O2 sensor / fuel pressure regulator ?


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Hi folks

I have a fuelling problem with my SE.

It seems to be running incredibly rich and has awful hesitation / stumble when under load during warm up and, to a lesser extent, when at normal running temperature and idle during warm up is a little erratic and always seems to be hunting (more so than I would expect).

Having done some Freescan logs, the O2 sensor bounces from 0.2/0.3 up to 1.1 which, in itself, indicates a rich mixture (I previously replaced the O2 sensor with a genuine Lotus item) but the running problems during warm up and in Open Loop mode would indicate that the O2 sensor isn't necessarily the cuplrit (particularly as it was replaced and made no difference unless I bought a duff genuine one !!). Also, to note, there are no fault codes showing in Freescan.

So I went looking at other possibilities. MAP sensor pipe is fine and the MAP readings seem normal. All other sensor readings seem pretty normal. Checked the other vacuum / pressure lines and they all seem OK.

I then went on to checking the fuel lines. It could be old injectors with poor spray pattern, fuel filter needing cleaning etc which are yet to be investigated HOWEVER I wondered whether the fuel pressure regulator is shot and hence too much fuel is getting in as a result....

I don't have a pump/gauge at my disposal to check the pressure but I did try removing the manifold pressure line from the regulator whilst the car was idling and it made no difference to the idle whatsoever.

My initial question is should the idle change on removing the manifold pressure/vacuum from the fuel pressure regulator ?? If it should change and mine doesn't, is that a fair indicator that the pressure regulator is shot ??

If it shouldn't change and my car behaved as it should during this test, does anyone have any ideas / pointers for me to look at next, bearing in mind the checks I have already made ??

Your help is much appreciated :getmecoat:


Edited by SteveSmith


89 SE Pacific Blue

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Bible says :

Fuel pressure too high, diaphram could be shot - check for any fuel in the vac line

Leaking injector,

Charcoal canister full or in need of purge,

MAP sensor reading too high fooling the ecu (already checked this though)

TPS accuracy

02 sensor, and or wiring

I have the opposite problem on the GT2, so I'm gonna take some of my own advice and check the list in the bible soon as well.

You can write a few off of that list already, leaking injector sounds good, regulator could well be buggered but my overall bet is the o2 sensor.

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Cheers Jono

Having previously (admittedly a while ago now) changed the O2 sensor (with a genuine Lotus item) and it making no difference together with the fact that it is impossible to drive the car during warm up and hence in Open Loop mode then surely this would exclude the O2 sensor from my investigations ?!?!

Another thing I forgot to mention is that the fuel pump relay is getting rather warm. I've swapped it out with another relay but that gets warm too. Is it meant to ?!?! Or would that be another useful indicator towards diagnosing the problem ??

Edited by SteveSmith


89 SE Pacific Blue

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If it's getting warm it has too much current through it or it's clicking on and off very quickly or it has a bad contact causing low cross section of the conductor and hot spot which is thermal conducting into the relay + wiring.

Check the wiring around the terminals (usually they break off leaving 2-3 strands to do all the work = heat)

Check the relays are not low resistance (will cause too much current to flow)

The relay serves the pump and the injectors themselves - if the wiring is warm / damaged then I persume a problem but I doubt it.

I also wonder if your sparking leads are connected correctly...

Obviously lean mixture, harsh stumble.....could be spark plugs, leads, coil, injector, air filter.

When I used to be a prob shooter with engineering jobs with wide angle problems I used to list all the probables vs the time to fix vs their cost vs their liklihood of solving the problem.

EG - spark plugs easy fix and cheap so do these 1st, they're also consumable so they'll need replacing one time...

I dunno your maintenance schedule was last so probably 1/2 of these could be done already.

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Wooooooohoooooooooo my problem is resolved..... Thing is, I'm not sure which of a few things I did actually fixed it :wallbash:

So here's what I did...

Checked (removed and refitted) all the vac pipes from the inlet manifold

Replaced the fuel pump/injector relay

Checked all electrics going to the ECU (including 2 dodgy connectors that a previous owner had fitted to the light green wires in the relay box in the boot which seemed to be getting warm so recrimped them)

Personally I reckon it was the dodgy connectors but I haven't studied a wiring diagram yet to figure out what the light green cables actually serve - they seem to enter the relay box with the rest of that section of loom but then double back on themselves without actually doing anything in the relay box which is very odd.

Anyway, took the car for a spin from cold and drove it around for half an hour and it behaved perfectly with no stumble/hesitation at all. In fact it hasn't performed this well since I bought it 6 years ago :police:

Only thing I noticed from the Freescan log is that I only seem to be getting 91% on the TPS despite planting my foot to the floor so there's more power to be had.

Chuffed as anything here. I'm loving my car once more :whistle:


89 SE Pacific Blue

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At only 91% you're missing out on some real fun stuff, get that TPS adjusted :)

88 Esprit NA, 89 Esprit Turbo SE, Evora, Evora S, Evora IPS, Evora S IPS, Evora S IPS SR, Evora 400, Elise S1, Elise S1 111s, Evora GT410 Sport

Evora NA

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  • 4 years later...

Hi guys,


My 1998 GT3 is running rich causing hesitation after 15 mins.


We have checked the following


Plugs + plug connections

Fuel pump

Fuel filter

Air filter

Vacuum pipes


When we removed the O2 sensor plug, it made no difference and the the engine management light did not come on.


Any thoughts on where to look next?





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How did you check the pump?


Generally removing the O2 sensor should do something!  So that may be your clue.


You should really log some freescan data. If you can get a hold of freescan and a cable, buy the cable or borrow from another Lotus Esprit owner.  Then you can email me the data.


If you have freescan, start logging data with the engine cold, let it idle up to fully warm at ~80C, don't touch the throttle the whole time, then take it fora  drive through all the gears, use boost. then send the file to me.


Vulcan Grey 89SE


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Thanks for getting back to me Travis.  We changed the fuel pump in 2010 so we didn't actually check it formally (good point -thanks!).  I don't have freescan myself so will try and borrow it from a friend of mine.  


Does the Esprit actually have an automatic choke? 


Thanks in advance.



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The Esprits 89+ all have an idle controlled by the ECU via the IAC idle air control stepper motor. No choke.

If you replaced the pump, then what did you do for the pulsation damper above the pump? Did you re-use it or replace it with a rubber hose? Either case, that may be a source of pressure loss, especially if you used the wrong type of rubber hose...

The non-fuel submersible rated hoses will split after a year or so.


Vulcan Grey 89SE


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Things to check:

Coil Packs - normally check for 2 and 3 - as they appear to be the ones that get the worst weather

Weak Injectors - they can do some funny things - they can appear to be working - but actually not - does it ever miss?

Not sure how long you have had the car - have you checked the plugs are correct?

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