Interesting Car Issue-VW Tiguan

[VaTom]

A friend of ours. Single woman owns a VW Tiguan about 9 years old. Takes care of her car but not a car person. She added a quart of oil a few days ago. Warning light came on next day and car starting running very rough. She had it towed it to VW dealership. Called me for advice. Thought it might be PVC valve issue.
Service tech found out she had not put oil fill cap back on properly so that the necessary vacuum was not created for PVC valve to work properly. This caused the car to run rough enough she couldn’t drive it. Never heard of this happening but looked it up on line and it was a correct diagnosis.Assume it could happen to other makes and models.

 

[glowe]

I am certain that the mechanic was correct. I have changed my own oil for over 50 years and finally had my first issue this last summer. Apparently, I did not properly tighten the oil filler cap when doing my last oil change. I found that my car hesitated and seemed to lose power when climbing hills. Got home, opened the hood and there was no oil filler cap, but some oil on the underside of the hood. Went to part store, bought a new cap and problem was solved.

 

[lkabug]

My daughter had same issue with her VW. However, her oil changed was done at the dealer. After a few miles she drove back to the dealer's garage and the oil cap was found on the mechanics tool chest.

 

[yeti]

Yes, it's a thing. Vacuum leak can cause problems like you (OP) referenced. The PCV or air-oil separator will not operate properly if there is a vacuum leak.
The PCV is a workaround to keep the catalytic converter at a reasonable temperature range, but can be a PITA at times.

 

[H Richard]

Yes, it's a thing. Was nearly 10 years ago and my mechanic's helper didn't quite get tie cap on tight and car (Toyota Highlander), ran rough and I returned it to the shop within minutes. The Owner knew immediately what was wrong as had just been reading about the problem in one of his bulletins. He hadn't discussed it with his helper yet.

 

[lihpster]

'98 Nissan Frontier. bucking and dying, flooding and starving. Had to buy a code reader to find out what was wrong. Turns out it was a mass air flow sensor. A small piece of plastic and wire. Costs maybe $0.10 to make. All it does is measure the density of the air going into the throttle body so the computer can adjust the fuel mix so the truck runs "perfectly". Of course, a carbed engine from 1973 didn't have that problem. Here's the kicker: they don't sell mass air flow sensors by themselves. You have to buy the whole throttle body. $400 later, my truck worked. Because of a $0.10 part.
That's the issue with today's cars. They work great until 1 sensor goes out and confuses the computer and the whole thing stops working.

 

[bobsguns]

'98 Nissan Frontier. bucking and dying, flooding and starving. Had to buy a code reader to find out what was wrong. Turns out it was a mass air flow sensor. A small piece of plastic and wire. Costs maybe $0.10 to make. All it does is measure the density of the air going into the throttle body so the computer can adjust the fuel mix so the truck runs "perfectly". Of course, a carbed engine from 1973 didn't have that problem. Here's the kicker: they don't sell mass air flow sensors by themselves. You have to buy the whole throttle body. $400 later, my truck worked. Because of a $0.10 part.
That's the issue with today's cars. They work great until 1 sensor goes out and confuses the computer and the whole thing stops working.

A junkyard is a good option for things like this, IMO. In fact, I would try to score two at once in case one dies too soon.

Some older Toyotas have that stuff & Toyota doesn't sell the part anymore. Toyota used to run ads on their trucks going for 500K or 1mil miles. Now they pull this garbage.

 

[lihpster]

A junkyard is a good option for things like this, IMO. In fact, I would try to score two at once in case one dies too soon.

Some older Toyotas have that stuff & Toyota doesn't sell the part anymore. Toyota used to run ads on their trucks going for 500K or 1mil miles. Now they pull this garbage.

Had a friend who worked at a garage get me the part for cost. I did the work myself. Didn't bankrupt me but the time and effort I had to put in really chapped my hide.

 

[Oldengineer]

Where do I start? The Positive Crankcase Ventilation valve meters airflow across the crankcase proportional to load. The Higher the load (throttle angle), the more flow. This airflow is essential to 1. Keep the crankcase pressure negative to keep the crankshaft seals from blowing out, and 2., extend engine oil life by burning combustion blow by. The PCV valve uses manifold vacuum, and represents an unmetered airflow that the Mass Airflow sensor doesn’t recognize (the MAFS provides the KEY variable for the calculation of the fuel injector pulse width). If the oil fill cap is not sealed, it is a vacuum leak —- kind of like having a hole in the intake manifold. The engine is going to (try) run lean since the leak is after the MAFS. The PVC valve is an engine certification component and is specific to the given engine family. It is also an engine idle speed (think fuel economy) variable since it provides some of the idle airflow. Attempts to improve the vehicle FE label by lowering the idle speed —— and reducing the PCV flow is a very slippery slope! I believe that Toyota had early engine failures due to this practice.

There’s a lot more to this, but leaving the oil fill cap loose….or off, upsets the fuel/air calculation just like leaving the gas tank fill cap loose affects the evaporative emission control system. It seems like tightening the oil fill cap should be a no-brainer. Maybe not for some.

Tom H.