1999.5 ALH Jetta intermittent idle stumble

[RhysOnArrakis]

Hello, I am hoping to get a few good opinions here on this issue that I am having. Engine will idle fine, then will act as tho misfiring for about 5 seconds, then resume normal idle. This cycle repeats anywhere from every 10 seconds to every 30 seconds, no matter if the engine is cold or warm, after a drive or initial start-up. Above idle all is normal, runs very strong. It's been doing this for a few months now, and as this is not my daily I planned to troubleshoot this time permitting. But I have no failures or DTC's per VCDS, and even a tick above idle and the problem goes away! So far the most likely culprit in my limited TDI experience is a sticking fuel injector. Any other input would be greatly appreciated. 269,000 miles, fresh EGR overhaul and intake cleaning, ALL vacuum lines replaced, all regular servicing done religiously. Thanks!

Rhys L.

 

[Nero Morg]

Sounds more like you're getting air in your fuel system. Common leak points are the thermo T on the fuel filter, and the quick disconnect lines next to the strut tower.

 

[IndigoBlueWagon]

If you have VCDS I'd try raising the IQ (injection quantity) a bit. '99.5s experience what some people call a "slowdown shudder", especially when they're cold. It can start to appear as injectors wear and deliver a small amount of additional fuel. Bumping the IQ up a bit usually fixes it.

 

[Prairieview]

Original lifters?

 

[RhysOnArrakis]

Original lifters. And while I was in looking for leaks in the under-hood fuel lines, I couldn't believe what I saw (heard). What I thought was a valve cover gasket leak was a growing diesel stain. I shut off the engine and heard hissing from #2 and 3 injectors. These injectors need to be re-sealed, yes? Just pull all 4 and put in new seals while I'm at it? After that I will start exploring the higher functions of vcds.

Rhys L.

 

[Nero Morg]

All the fuel supply lines from the pump to fuel tank are under suction. If you have clear lines you could see bubbles for air intrusion. As for hissing noises, spray soapy water at your injectors/glow plugs where they meet the head to see if you have compression leakage.

 

[RhysOnArrakis]

Definitely injectors 2 and 3 leaking, confirmed with soapy water test. Going to get those sorted and then start changing some 20 year old fuel hoses!

 

[RhysOnArrakis]

Hello all, back again and still struggling. I pulled all of the injectors, cleaned them very well, and re-fitted with new copper seals. Those leaks are confirmed fixed with the soapy water test. All of the fuel lines were checked back to the fuel sender at tank, and the rubber lines from firewall to filter - filter to HPP I just replaced with new VW parts. Also changed the filter (Mahle), filter T, and all the spring clamps. I filled the filter with LiquiMoly Diesel Purge because, why not? Started right up, and after about five minutes I checked for bubbles in the new clear feed line to the HPP. Not even a hint of a bubble. Yet, the stumble continues. At least now I can rule out the fuel feed system. I will be going into the tank next after the new sender gasket arrives, might be contamination in there and would like to rule that out also. Any idea on what to check next? There are no faults being logged and I'm still a noob at VCDS, but a quick learner. Thanks to all in advance!

 

[RhysOnArrakis]

I will try looking into that, however I would guess that not to be the case as the problem is intermittent. It's like a light switch, shudder one second, perfect smooth idle the next. Rinse, repeat at random intervals.

If you have VCDS I'd try raising the IQ (injection quantity) a bit. '99.5s experience what some people call a "slowdown shudder", especially when they're cold. It can start to appear as injectors wear and deliver a small amount of additional fuel. Bumping the IQ up a bit usually fixes it.

 

[burn_your_money]

If you have VCDS I'd try raising the IQ (injection quantity) a bit. '99.5s experience what some people call a "slowdown shudder", especially when they're cold. It can start to appear as injectors wear and deliver a small amount of additional fuel. Bumping the IQ up a bit usually fixes it.

I'd start with this.

 

[RhysOnArrakis]

I'd start with this.

Will do, I'll see what happens!

Rhys L

 

[RhysOnArrakis]

You guys nailed it, fuel IQ was at 0.5, raised it to 3.0 and the idle is now as smooth as glass. Full beans was 36.0, just what the book says. Thanks all for the great advice!

Rhys L