Perkins has uneven idle at operating temp, but consistent when cold.

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fergycool

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I have two Perkins 4.270 on my boat. Before they have reached operating temp (80degC thermostat) both idle at 750rpm. Normal cruising speed is 1,300rpm.

However, once operating temp is reached the port engine will idle very unevenly. The tachometer will show varying rpm of 100 to 800. It's accompanied by a flickering low oil pressure light (cutout of 20PSI) as well although the oil pressure gauge never drops below 30 PSI. I guess the cause of the low oil pressure is just due to the low engine speed.

The starboard engine is fine and rock solid at 750rpm.

Above 1,000 rpm both engines perform identically.

Both engines take fuel from the same tank (not ideal I know).

I have tried adjusting the idle screw. I temporarily upped it to 900 rpm. Once again at cold it idled at 900 rpm but once up to operating temperature the same symptoms returned.

My first step is going to be to replace all fuel filters (primary and secondary). Any other tips, comments?

Thanks.
Ferg
 
It could be a tiny fuel leak letting air in somewhere and the symptom of varying revs goes away when the engine is at cruising revs and more fuel is being sucked through. Put talcum powder on all the joints, bleed screw and any fitting at the bottom and top of the filter. Run the engine at lows revs for at least 5 minutes then check the talcum powder for diesel. As air is sucked in a small amount of fuel leaks out, diesel will go through a microscopic hole. If that finds the problem please let us know.
 
Thanks a lot for those two comments.

That's given me some room for thought. I'll check to see if the injection pump is readily available first! Probably not....!

Cheers
Ferg
 
It's not that common a Perkins I'm afraid. 4.270 from 1960. Apparently shares many parts with the earlier L4 and the later 4.236.

It appears to be a CAV DPA pump.
 
If my Greying cells are ok, I seem to recall that this engine has a rather large oil pressure switch with 2 terminals one for oil pressure and the other one for stopping the alt. from firing up until pressure has been achieved. ?

The oil side connected in such a way that if pressure drops the stopping solenoid operates thus shutting the engine down. If I am correct then I would temp. connect to the good engine and see what happens.

There again I have seen many of these engine with this function altered when fitted in the marine environment certainly fitted when used for plant operation. Worth checking to see what you have got fitted.
 
Did this come on suddenly?
I'd probably check valve clearances, may have a tight gap.
You could crack off injectors one by one just a tad to isolate if it was a misfire on one cylinder.
I'd be surprised if was a fuel injection pump fault, it could be a lift pump and the diaphragm was getting a tad tired.
 
pampas said:
If my Greying cells are ok, I seem to recall that this engine has a rather large oil pressure switch with 2 terminals one for oil pressure and the other one for stopping the alt. from firing up until pressure has been achieved. ?

The oil side connected in such a way that if pressure drops the stopping solenoid operates thus shutting the engine down. If I am correct then I would temp. connect to the good engine and see what happens.

There again I have seen many of these engine with this function altered when fitted in the marine environment certainly fitted when used for plant operation. Worth checking to see what you have got fitted.
Click to expand...

Although there were apparently marine versions of this engine, my two are tractor ones that were marinised in the early nineties.

There is only a single terminal on the oil pressure sender. Originally there was no oil pressure switch but I added a T piece to extend the sender away from the engine mount as it was rubbing slightly. I added the oil pressure switch (as well as the original oil pressure sender) to fill the extra hole that adding the T-piece created.

There is no stop solenoid just a cable attached to a lever on the fuel pump.
 
john_q said:
I have a 1984 4236 and had uneven running recently so my mechanic suggested changing out the lift pump as we had a spare. Problem sorted.

A clue The old pump felt "weak" when you tried to use it
Click to expand...

Thanks. Apparently this engine shares a lot in common with the 4.236 and the earlier L4.
 
Vonasi said:
This wouldn't explain why the engine is fine at over 1000 RPM once warmed up. A weak pump would cause poor idle whether the engine was warm or cold and a bad pump would cause poor running throughout the speed range with surging and/or stalling. The OP could always try swapping the fuel pump from the good engine just to eliminate it as it is a smaller job than the injection pump.

My process would be to start with the simple stuff to eliminate anything strange going on. Check and tighten all fuel supply line connections. Remove and refit all fuel filters and ensure tightened and sealing properly - re-bleed system and test. If still faulty swap over the fuel pumps. If still faulty then remove injectors and have them serviced and cleaned. You'll benefit from this anyway even if they are not the fault. Basically you are trying to find anything in the hope that it is not the more expensive and difficult to change fuel injection pump.

If the engine has the cut off system that a previous responder mentioned then this sounds like another good thing to check out and bypass if fitted. Basically a process of elimination starting with the cheap and easy and working up to the expensive and more complicated.

I wouldn't be too sure about the possibility of it being a valve gap issue as it seems odd that it is only at idle that the rough running occurs. A valve held open would not close up just because the engine speed increases. But hey check the gaps anyway as it isn't too big a job.
Click to expand...

That's a thorough process and I think I'll do as you suggest.

It's due a fuel filter change. Should I do that as well, or is it best to not change much until I've sorted my problem?
 
paddywackcocker said:
Did this come on suddenly?
I'd probably check valve clearances, may have a tight gap.
You could crack off injectors one by one just a tad to isolate if it was a misfire on one cylinder.
I'd be surprised if was a fuel injection pump fault, it could be a lift pump and the diaphragm was getting a tad tired.
Click to expand...

I think that this has been happening for a while. However, I've only recently discovered it due to the following:

(1) The thermostat was stuck open and the engine was taking a long time to reach operating temperature before. I replaced that 3 months ago.
(2) It's only recently that I've been doing longer cruises. With the fixed thermometer the engines still only reach operating temperature after about an hour.
(3) It was the low oil pressure light that really alerted me to the low revs (am I blind?). I only fairly recently fitted these due to the afore mentioned rubbing sender.

I should also state that although pretty old, the engine does run pretty smoothly, apart from this one issue. It starts within two seconds. It smokes for 20 seconds or so after starting but is then clear. Oil levels are pretty consistent as well.

Hmm, one symptom that has only just occurred to me is that there's a small amount of unburnt diesel in the exhaust when it's idling at operating temperature for more than a minute or so.
Is that relevant?
 
As you are well aware you have very old engines, which were a kind of interim between L4 and 4,236. These early engines have hydraulically governed CAV DPA fuel pumps unlike later versions of 4,236 which had the far better CAV DPA with mechanical governing.

The characteristics you describe are of a hydraulically governed DPA simply pulling air.
 
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