Engine Idle problem

[desilva]

Hi, does anyone know why my engine (1978 SC) idles at approx 1500 rpm when Hot ?

When started from Cold the engine idles at approx 1600rpm for 5 mins then reduces to approx 1000rpm when warmer. After a good run the engine sometimes stays at 1500 rpm. Any ideas would be appreciated.

Thanks Phil

 

[Guest]

Hi Phil,
I sounds like your warm up proceedure is working OK at start up as the revs are at 1500 falling to 1000. You said sometimes the revs stay at 1500, what are the revs at the other times 1000?
It could be that the throttle is occasionally not closing properly, does this situation change if you blip the throttle when the tick over revs are high?
Cheers
John

 

[bones]

Everything that John said [] but also has the car been serviced recently? ie are your valve clearancec correct, is the mixture strength correct? The sensor plate in the throttle body can also stick, sometimes idle speed is set high by a previous owner to compensate for poor idling at the correct idle speed of 950+- rpm. Get you mixture checked out on a proper machine before fiddling with anything else though or things can get expensive quickly.
One quick way to check if the idle speed is set deliberately high is to get the engine to the correct running temp' and remove the oil filler cap, if the engine speed doesn't lower and it runs a little rough then chances are the engine speed is set deliberately high.

 

[desilva]

Thanks for the advice, John and Bones.

When bliping the throttle the revs do not change and the engine still idles at 1500rpm. Occasionally when comming to a stop it will idle at just below 1000 rpm but as soon as i blip the throttle the revs rise and stay at 1500 rpm. I can get the revs down by letting the clutch out slowly in gear when standing still, the drag pulls the revs down !

I have tried removing the oil cap when warm as suggested. There is no change to the revs and the engine still runs smooth.

The engine had a full service Incl valves 4 years ago but has only done approx 4000 miles since. Each year since the oil and filter has been changed. In 06 the injectors, fuel distributor, fuel pump and warm up regulator have been replaced to cure rough running and poor hot starting. All other service items eg plugs and filters are in good condition. The car runs great and always starts easily cold or hot.

I think i have to "bite the bullet" and get the mixture and idle settings checked out ! any reccomendations in the Warwickshire area ?

Thanks Phil

 

[bones]

I can't recommend anyone in your area but from what you said try getting hold of the throttle linkage in the engine bay next to the throttle body and operate it manually, just to check for sticking as John pointed out, there are quite a few places the linkage can snag including on the gearbox, (easy to get to). However , the fact that the engine noise doesn't in anyway change when you remove the oil cap siggests the idle speed might be tweaked, in which case it is off to an Independent for a check up. Watch yourself around the engine fan with the engine running if you are not used to being in there.

 

[desilva]

When i removed the oil cap the engine speed may have lowered very very slightly. I will check out the throttle linkage manually.

Thanks again

 

[Guest]

Hi Phil,
I agree you should get the checks you propose done.
Also don't forget what Pete said re valve clearences , as you have had new valves 4000 miles ago then this is a good idea to have the clearences checked. If you need any tips checking the valve clearences your self let us know.
Cheers
John

 

[t11ner]

Wouldn't the idle speed altering when you remove the oil cap be an issue of mixture rather than speed settings?

Steve

 

[bones]

The point is, if the idle speed is set to its correct tickover speed of 950+- when you remove the oil cap the idle will slightly falter due to the vacuum pressure change in the whole sysytem. One way of telling if the engine's idle speed is set artificially high DELIBERATELY is when the engine doesn't falter slightly after the oil cap is removed. This would then signify the the high idle speed was so set to mask another more expensive problem.

 

[Guest]

Phil,
Going back to your earlier post , did the engine idle properly after the major service?
If so when did the issue start, was it recently?
Have you tried Pete's suggestion regarding disconnection the throttle and operating it manually?
It's always best to check the easy things first before putting your hand deep into your pocket.
Cheers
John

 

[t11ner]

ORIGINAL: bones

The point is, if the idle speed is set to its correct tickover speed of 950+- when you remove the oil cap the idle will slightly falter due to the vacuum pressure change in the whole sysytem. One way of telling if the engine's idle speed is set artificially high DELIBERATELY is when the engine doesn't falter slightly after the oil cap is removed. This would then signify the the high idle speed was so set to mask another more expensive problem.

I don't see how that proves it was set high deliberately or that it was done to hide another fault. Removing the cap can be an indicator of the fact that the engine is set up and running correctly but only because it would weaken the mixture by allowing atmospheric air in via the breathers; this weak mixture will alter the note of the engine.

Whether the idle is set high deliberately or if (for instance) the butterfly or auxilliary air valve are sticking open slightly, the result of removing the oil cap would be the same and so would prove nothing about why the speed is high, whether it was set that way deliberately or whether it was done to cover another fault. Most likely, the reason that it makes little difference at the higher rpm is that the amount of air entering via the breather is relatively small compared to at correct idle and the engine is less sensitive to mixture changes at higher revs.

 

[desilva]

Hi

When i said valves were done at last service i mean clearances and not new valves. Yes the engine was ok until the recent warmer weather. Having inverstigated further the engine note does change when i remove the oil cap. If i can get the car to idle approx 1000 rpm (in gear drag clutch get out and remove the oil cap !!) the revs decrease to about 950rpm then increase back to approx 1000 rpm when i replace the oil cap, if i try to blip the throttle by hand via the linkage the revs rise to 1500rpm but wont decrease even when i push the linkage closed. There does not seeem to be any snagging of the throttle likage or cable.

I have read in the Haynes manual that a cause of high rpm when warm could be the auxiliary air valve, the only problem seems that it is very very difficult to get to !! any suggestions ?

Phil

 

[t11ner]

Aux air valve wouldn't normally drop with clutch drag and then stay down when the clutch is dipped again, and as discussed above I'd suggest that dipping when the oil cap is removed only indicates that the mixture is in the right ballpark.

I'm wondering if the centrifugal bob-weights in the distributor might be sticking in their advanced position causing the fast idle. When you rev it they spread out to advance the ignition timing and they should be pulled back in by springs as the revs drop but if the springs or pivot points are worn or sticking then the weights could hold open causing the problem. By dropping the idle speed with the clutch, you are reducing the centifugal effect on the weights which may allow them to close.

Can you get hold of a timing strobe? If so just check the timing at idle and fast idle and see how it drops as you release the throttle and as you drop the revs with the clutch; if it drops, say 20 degrees or more on the last 3-400 rpm (vac disconnected from dissy) then you might be in the right place. If this is the problem it might be curable by simply adjusting the spring tension on the bob-weights - if that doesn't make the difference then it's a new dissy.

Alternatively, check if you can access the bob-wights and wire them closed then see if it still revs high - if it doesn't then you have proven the fault.

Steve

 

[bones]

Only trying to help, I've owned my SC for twenty two years I've probably forgotten more about it than I remember. [:-]

 

[t11ner]

Sorry, didn't mean to flame [8|], just thinking out loud on the problem [&:] I've been tuning and problem solving on engines for not quite 20 years now and still enjoy trying to figure out a problem []

 

[desilva]

Thanks Guys,

More stuff to try. I will dig out my old strobe, last used at least 15 years ago !

Phil