Intermittent stall/rough idle

Before I start, thanks very much to the knowledgeable and generous forum folk who have helped me so much during 12 turbulent years of Jimny ownership!

Looking at the intermittent stall and rough idle threads produces a long list of suspects IAC, EGR, PCV valve, vacuum leaks, sensors etc, so trying to get some gut feelings before starting the process of elimination.

2006 JLX VVT automatic, 189,000 kms, started occasional stalls at traffic lights whilst at operating temperature, just quietly died but then restarted and ran absolutely normally. Seemed maybe a bit rough/low on power when driving (but only once and I’m not 100% sure).

My mechanic guessed fuel pump so put in a new pump and filter but the problem was still there first drive after getting it back but then no issues for 5 weeks.

Then last week it started (from warm) but was running incredibly rough, engine shaking the car and on the point of violent stall under 1000 rpm but came good with a blip of the throttle until the revs dropped down again. Turned it on an off a few times and the same issue but then after waiting 40 mins it was normal and drove it home.

My mechanic suggested electrics so I took it to a specialist but after 2 test drives and letting it idle for 45 mins the problem didn’t show, so they didn’t do anything.

Only possible common thing is that since the new fuel pump, it’s only done it at/after low speed speed bumps (whilst queuing) or when parked after a high curb.

No engine warning light at any time (so not sensors?).

No abnormal noises.

Basically no issues when running (apart from maybe once) only at idle when warm.

Idle is maybe bit slow at 600 rpm, dips lower when activating electric windows etc.

Rocker box cover oil leak seems to have got worse recently (getting pressurised?).

PCV valve seems fine, rattles, activates when I suck it, cleaned it but it’s 12+ years old so obviously a new one will make sure.

Had some electrics checked out 6 months ago after the main battery fuse blew, battery and alternator are under 2 years old.

Gets injector cleaner in the fuel every 6 month service.

Thanks very much in advance for any insights.
 

My suspect would be the EGR valve not closing completely, and here's the reason why.

EGR, or Exhaust Gas Recirculation is used to lower NOx emissions by introducing an "inert" or "non-combustible" gas (the exhaust gas should not have any oxygen), which reduces cylinder head temperatures.

The EGR valve opens to allow the intake vacuum to suck exhaust gas from the exhaust to the intake at specific times during vehicle operation, one of them being when the engine is on the "over run", you take your foot off the gas, the throttle is closed, the car in gear and the forward motion of the car is actually what's keeping the engine turning. When the engine is on the over run, the EFI system also stops fuel injection, known as "fuel cut".

The scenario I just described is quite common as you're approaching a traffic light, you see the red light ahead, you lift off the gas and allow the vehicle to slow, maybe down shift, and eventually clutch in as you brake to a stop.

As the rpms drop to idle, the ECR should close and the EFI system start injecting fuel again, but if the EGR doesn't close completely, there's insufficient oxygen to support combustion and the engine dies.

When you restart the ECU cycles the EGR valve (it's motor driven) to ensure it's closed, and in most cases, this allows a restart as if nothing was ever wrong.

When you're approaching a speed bump, the drive sequence is going to be quite similar to that as you approach a traffic light and the EGR is not enabled when the engine is not up to operating temperature.

You've got maybe three or so of the symptoms of an EGR valve being dirty and not closing properly, I'd say it's worth taking a look at.

The other common thing that causes stalling when warm is either or both the cam and crank sensors being tired but not fully failed. It's also not unheard of for the cam sensor to be damaged when changing the alternator as it's right where a bar might rest unde the alternator when the belt is being tightened.

Hi All 

I have a similar issue with my 2001 manual Jimny, however my issue only arises when the engine is cold.

Would the EGR valve still be a likely cause even when the engine hasn't warmed?

Is this something that is easily available to buy or am I likely to have to look for a second hand one somewhere?

Thanks again

Fordem and Lambert, thanks very much for the suggestions and explainations, it's greatly appreciated and everything you've both said makes a lot of sense and fits very well. 

Work is in progress, I'll update the thread for members' future reference when I know more. 

Thanks again.