V8 Slow Idle Decay and Minimum 5th Gear Speed

[Guest modifiedv8engines]

Hi All

 

My V8 has always had a slow decay to idle speed so when stepping off the throttle, and say at rest/still the engine takes sometime to settle down.  

 

I know this is a feature of the design and may have been engineered this way to prevent trans issues etc etc

 

However just been out for a short run and in 5th gear the slowest road speed acheivable is approx 33 - 34 MPH.  

 

So in order to keep within the the 30 MPH speed limit, 4th gear has to be selected and even 3rd on those 20 MPH sections.

 

Interested to here what other V8 owners find with their cars ?

 

Thinking my car may have an air leak somewhere or leaking injectors or someone has installed a chip at some time

 

Fairly cold weather (5 deg C today) but car accelerates like a rocket and much more than half throttle in 3rd induces wheel spin. Yeah tyres are not that warm but car is very quick 

 

Any comments most welcome

[Günter]

918 V8 Esprit's do have a notable 'long gearing'  ..so it is a runner, even on 'idle' in 5th gear

 

My experience is this, with 5th gear selected and without the loud pedal pushed it runs 40km/h without stalling

[t70]

Are you having other Idle problems? Is the car having trouble settling to idle normally? Does the car do the same thing with the clutch in while driving?

[WEllison]

I have a 2000 V8 and have never have this kind of high idle speed problem. Yes, the decay time is longer than I would like (maybe 1-2 seconds) but I've never had the 5th gear speed problem you are experiencing. If you have a scan tool, the IAC percentage should be about 30-35% after the decay.

[redshift]

Hi Paul,

 

With mine it has this 'idle-up' feature that gives around 1200 rpm once the car is moving. Can't remember what that equates to in speed but 30 mph wouldn't surprise me.

Cold mornings are wonderful aren't they, just need some tire warmers now

[Guest modifiedv8engines]

Thanks for the feedback and comments guys

 

On taking a second, third ,forth drive ( Yes - low temps and on boost = exciting driving) also noticing the revs are hanging up  when changing in the lower gears. So it seems part or no throttle gives a problem but when using half to full throttle, rpm drops when off the pedal   

 

Bellcrank which operates TB appears not to work at a linear rate so small throtle openings require little pedal effort and more for WOT...

 

Do the easiest thing first, so will check out the throttle cable as this may be a bit sticky or gunged up from sitting for about 6 months and not being used.  

 

Also found another problem but a minor one... missing throttle cable to cam cover "C" clip = cable shifting which cant be helping this issue. Clip obsolete from lotus but just ordered one and a spare from SJ

 

Not particularly keen on the slow idle/decay rate anyway as in some cases and at low speed in high gear, the car needs to be driven on the clutch and brakes - did find a thread where there was a mod to fit a secondary solenoid to resolve this, but this may solve one problem a but create another

 

Paul  

[redshift]

There's also the possibility of putting a relay in the IAC control signal path. This could be disactivated by a manual switch or a microswitch on the clutch arm depending on the effect you want to achieve.

Just need to remember to activate it on startup and when using AC.

[Guest modifiedv8engines]

Todays attempt at solving this issue

 

Adjusted throttle cable so that there was slack in the cable ( full throttle therefore not available) and engine rpm still hanging in the lower gears when changing gears.  

 

Would appear to be the throttle position sensor or one the other things that I have not changed in the past 3/4 years of ownership.

 

Looks like this sensor can be changed without removing the intake plenum which I hasten to add, I could probably change in the dark, blindfolded and standing on my head as its been off that many times.

 

No fault codes recoreded for this item or for any other

 

Stumped again  

[t70]

I would bet that your iac is dirty.

[Guest modifiedv8engines]

Thanks for your input Mark

 

Symptoms would kinda suggest the Idle valve could be at fault but this was replaced less than 500 miles ago. No guarentees I know but I think probably not this as idle speed does decay to 850rpm when car is parked.

 

My gut feeling is its a TB sensor, sticky TB butterflies or the ECM is not seeing/calibrating the parameters between part throttle and no throttle... if that makes sense.  

 

Have read on these forums about the ECM needing to learn the various idle settings etc etc. My car with new HD battery connected has never stalled which apparently it should during the 50 or so miles it apparently takes fo the ECM to relearn the new base settings.

 

Time to plug in my ancient old laptop with peters scan software again

[redshift]

Paul, I'm pretty sure that you will find that it is the ECM controlling the IAC that generates these effects. Remove the signal to the IAC and the revs drop like a stone between gearchanges. Hence my past playing with a microswitch on the clutch release arm.

[Guest modifiedv8engines]

Pete, Mark etc thanks for your responses.

 

Was under the impression that disconnecting the signal wires to the IAC would throw up a warning light and ECU would go into "limp home mode" ? 

 

This afternoon I tried this :

 

Start engine - engine cranks, starts and rpm @ about 1500. Engine Speed decays to 1200rpm and then after several seconds the idle speed drops to below 1000 ... about 850 or so.

 

Blip the throttle to say 4000rpm, on releasing throttle rpm drops rapidly to 1200 then flicks up to about 1300rpm before going back to 1200 and then setltles after several seconds to around 850rpm.

 

Does this sound familiar or is the 1200 and then 1300rpm telling me the IAC is faulty, dirty or playing up.... yet again  

 

Comments appriciated

Edited March 6, 2014 by modifiedv8engines

[t70]

There is an idle sequence.. As I recall.. idle the car from cold to hot. DO NOT TOUCH the throttle. Let the fans cycle 3 times with the A/C off and then 3 times with the A/C on. Then.. for some reason, I had similar issues to you.. I started it one day when it was about 70 degrees and hooked up the scan tool. It then went into an idle up and back down sequence (from 850rpm, slowly up to about 1600 rpm and then back down)... this solved my problem until the next time I disconnected the battery.

[redshift]

Never seen an IAC related fault light so it doesn't seem to be a closed loop function in the ECU software.

Same with IAC malfunctions, no indication either.

 

You start sequence sounds identical to mine. Might be different on blipping the throttle to 4k, cant remember the decay sequence but I have the impression that the revs hang up slightly higher than yours.

[Guest modifiedv8engines]

Thanks again Pete, Mark

 

Carried out the engine start and run until fans operate three times as advised and although the problem ( slow idle decay) is not completely cured, the engine seems to run much smoother and there is less of that infuriating increase by 100 rpm or so  when changing gear.  

 

Didnt bother with the a/c since I removed most of this when I replaced the coolant radiator as cooling always seemed marginal with another matrix placed in front of the cooling rad. Plan is/was to install an intercooler here at some time... perhaps.

 

Went out for a fairly gentle run approx 15 miles and the car seem sto be better and behaving itself. Must remember not to disconnect the battery again !

 

Thanks again

 

Paul