When sat in traffic, Engine runs shit and oil pressure drops

[Jason S.]

hi, my first post. Driving a ‘72 240z. Drives great when traveling along above 20MPH, but after operating temp has been reached, it hates sitting in traffic. RPMs intermittently drops way down an tries to stall. Oil pressure also drops. I can keep it running by heel toeing the accelerator while stationary, but it gets difficult the more the car is sat in traffic. Works perfectly, even in traffic for the first 15-20 mins after a cold start.

Any ideas? Carbs have rebuilt and serviced a few times but this issue has never changed.

 

[toopy]

Oil pressure when hot can drop surprisingly low on the gauge, it's entirely normal, if a little disconcerting!

When you say intermittently, is the engine hunting, as in, slightly faster than idle and then almost stalling over a regular time scale?

I had that on mine for ages, turns out it was because the carbs weren't perfectly balanced, they may each be tuned pretty good mixture wise, but it can be a tedious process to really get them working perfectly in sync.

 

[jonbills]

What sort of exhaust manifold and inlet setup have you got? All standard?
Posh exhaust with no heatshield and k&n filters or similar direct on the carbs can cause probs with the fuel getting hot in the carbs.

 

[Jason S.]

carbs were ballanced. i am thinking the fuel getting hot as well. stainless steel manifold and exhaust. but it did this when i had a regular exhaust as well. but it makes sense, because once i get up to cruising speed, (and cool thing down a bit) the issue goes away for a short while even for the next first couple of minutes i next get stuck in traffic.

what would be a good heatshield system? wrap the pipes.

thank you for the reply, its much appreciated.

 

[AliK]

I had this a lot!!! I once even caused a two mile tail-back braking down with this issue at the entrance of the Blackwall tunnel when I first got the Z and was idling in London Sunday eve traffic.

Definitely heat related. A few things affect this:

1. The timing on mine had been set way way too retarded by PO and a specialist after a full service / tune up!! So the fuel was finishing it’s burn in the headers making them very hot.

2. The mix being very lean at idle causes heat.

3. As already said, lack of adequate shielding.

My oil pressure on both old L26 and new L28 does the dance between less than a quarter to over three quarters (idling to on the move). As long as you see oil spraying all over the place on the cam when you take the filler cap off with the engine running, you are OK. Also clean up the electric connection to the sensor on the side of the block - I once plapped myself when running in a rebuilt engine and my gauge started showing low pressure and went to zero in traffic!!!! It was just the connector being slightly loose and corroded on the inside. Oil pump failures are very rare.

Final thought - a vac leak on carbs / gaskets can have you chasing you tail a lot so have a look at the condition of your carbs and gaskets too.

Let us know how you get on and if you fix it.

 

[Rob Gaskin]

Have you got the standard radiator fan on the car? These are very good and help keep the engine bay cool.

If you have an electric one make sure it switches on when needed.

 

[richiep]

As per Rob, I'd be thinking about cooling system efficiency. Z engine bays are hot places and if things aren't right, it can go south quite easily. Check whatever fan system you have, thermostat, rad, etc. How quickly does the hunting and stalling come on when standing?

One time-honoured cheat to help out - when sitting and the temp starts rising, turn on the heater and blower full blast. Yes, you'll sweat, but it should stop the engine temp building up.

The oil pressure dropping - normal. Freaks you out the first time you notice it!

Ali''s point about checking timing is a good one too.

An alternative to heat wrap on the manifold is ceramic coating. Yes, its expensive, but it works extremely well in my experience. I've had this done by Camcoat to both tubular manifolds I've run on my Z and I don't use a heatshield of any kind.

Also, cold air intake system can help - nice cool charge drawing air from outside the engine bay keeping combustion temps lower.

 

[Rob Gaskin]

Try running with your bonnet on the first catch and see if it makes a difference? If so heat is your problem.

Don't drive with the bonnet closed down but not on the catch it will open itself!

 

[uk66fastback]

Good tip Rob about the bonnet ... !

 

[Rob Gaskin]

Good tip Rob about the bonnet ... !

When I did trackdays I tried with the bonnet partly open but sometimes I was told to close it. So in the end on hot days I removed the inspection flaps.

 

[johnymd]

These cars are capable of sitting in blistering heat and extended traffic for long periods with no issues at all. Both my L-series cars are happy to sit for hours in over 35* C temps. On my Spa trip over the weekend my intake temp went up to 70* C and the engine never missed a beat, despite no tank return. So you certainly have a problem. Don't try to re-engineer things as it should work without extra changes.

Sorry this doesn't help or come up with an answer.

 

[AliK]

.

Sorry this doesn't help or come up with an answer.

Actually Johny, this is exactly what we all need to hear!! And something to aspire to! [emoji106][emoji106]

When I was running in the engine I wasn’t confident that I had the bonnet put back right and I was nervous about closing it fully for fear of it getting stuck. So as Rob said, I drove it on the catch all the way to Devon and back for the whole 350 miles!! No issues.

But it seems if all is well then you don’t need to worry! So let’s find that root cause.

On that note, have you flushed the rad / cooling system? Also what coolant are you running? Is it 50:50 glycol or something else?

 

[toopy]

In the past i have spent ages flushing through the engine and the rad, and the heater matrix, and it's never once over heated, i do have an electric fan, and it does cut in on hot days in traffic, not as much as you would expect though.

I did have issues for a while with the engine occasionally stumbling and then dying, and not always wanting to restart straight away, which initially i thought was fuel and heat related but was in fact a failing coil. Only happened on the move though, never when idling.

 

[jonbills]

Wasn't that running your EFI though?

 

[Farmer42]

FWIW I reckon it's a fuelling issue. It sounds like the heat from the exhaust manifold is either causing vapour lock where it starts running lean or the expansion of the carb body due to heat is causing an air leak with the same effect. A test would be to let it run up to temperature then whip out a spark plug and see if it is the right colour. If it's white or glazed then it's overheating due to lean fuelling.

As for air leaks check the gasket seals between carb & manifold. That's a favourite place.

 

[Jason S.]

thanks for all the replies. really appreciated. i have an after mkt rad that is more efficient than than the std and the engine temp is solid and runs according to whatever thermostat i put in. engine never over heats. stays at operating temp. i will check for leaks between carbs and manifold. i will look into heat shield. which, afterall, is not really messing with anything, just sheilding.

this car runs great on the motorway, had it up near 130mph, (recently put it on the rollers and it had not lost a single horse since new - my father bought it new and ran synthetic oils its whole life, and until the sitting in traffic issue occurs will idle at 350-400 no problem. (to prevent / reduce the stalling, i've had to crank the idle up to 900.) will get back with results soon. toopy, the coil was recently replaced with no change in the 'stuck in traffic' issue. again, thanks to all.

 

[jonbills]

Sounds like you've had it in the family a long time. Can you identify when The problem started?

 

[johnymd]

The silver car is running FI and no fuel return in the engine bay so that could be why it is not bothered by heat. The ratty car is on SU's (no return) and still copes with the usual 2hr Dartford crossing queues but does probably sound like fuel. Nissan new there could be a problem which is why they fitted an additional pump in warmer climates to keep the fuel flowing well. Maybe fit the rear electric pump to the existing wiring and enable power to it at the connector under the heater controls. You can just put a jumper across the connector to make it ignition feed.

 

[status]

I’d get the header ceramic coated as said before sounds like vapour lock,I had my nismo header ceramic coated which ain’t cheap and it cured exactly the same symptoms but I was running triple delortos,I thought the 240 had a heatsheild on it anyway

 

[Rob Gaskin]

Jason, we'd like to know more about the car and it's history please.

No wonder the oil pressure is low if you let it tick-over at 350-400 rpm. 900 is about right. Perhaps you are expecting too much from the old girl.

At 350 rpm it won't be pumping oil or water efficiently - or charging.