trev_dl

Have a 1986 Ninety Eight and am hoping some current members can offer some insight.

Whenever my car is in Park or Neutral the idle is a little rough. Whenever it is put into Drive or any other gear in motion the engine is silent and the car doesn't have that roughness.

Whenever I initially got the car the MAF sensor was unplugged and it was plugged back in (unsure why it was unplugged). I have a new one on the way but would like to hear what others think. Since the car operates flawlessly in every other setting I just find that odd.

I have replaced the EGR , IAC , air filter, and had the MAF sensor cleaned (waiting on the new to come in), and PCV.

My chassis service manual says other possible causes may be the MAF has failed completely (new one on the way), the TPS (new one on the way), or the Park Neutral Switch (new one on the way).

I have heard that sometimes a vacuum leak and cause this issue. A close friend of mine who has a history of automotive repair was extremely hesitant about touching the hoses if not needed as they are likely frail as well as the connections being brittle due to age.

So I am curious to know if others would recommend part replacement or going ahead and having all vacuum hoses replaced?

Many thanks in advance!

PS: It already has new spark plugs and wires as well.

1970cs

If you think you have a vacuum leak. Why not use carb cleaner on the hoses to see if have a problem? If so then it will pick up the RPM and smooth out briefly.

Pat

joe_padavano

As always, I would recommend troubleshooting BEFORE throwing my hard-earned cash at new parts that may or may not be required. While vacuum leaks are often the case when old hoses deteriorate and crack, and pro-active replacement is probably a good idea, rough idle caused by a vacuum leak nearly always gets worse when idling in Drive rather than Park. I suspect your car has a Park/Neutral switch that the ECU uses to control mixture and idle speed. If this is bad, it may cause a rough idle in Park.

trev_dl

@1970cs - I've heard of people doing this before. The carb cleaner won't hurt the vehicle? I have never done that. I am willing to give it a shot though for sure if it will help me in this quest.


@ joe_padavana - I had the same idea with pro-active replacement, I only receieved a small amount of the maintence records for that vehicle (when she passed there was so much to do it was mind boggling and the extra paperwork to go through was piles and piles and I wasn't always there) and I keep a record of every little thing. I figured that just due to age it would be wise to replace some parts to be proactive and that has come in handy a few times actually. The MAF sensor is a good example, it was unplugged and when my mechanic took it out to look at it the part was absolutely filthy (which surprised me as this car only got out once a month most the time when she had it). He cleaned it the best he could but even then still said it was in pretty rough shape.

@ joe_padavana - I didn't know that a vacuum leak tended to get worse in drive rather than in park. This is something I have been pondering actually. When the car is in drive it has plenty of power and the engine is silent and doesn't shake the car like it does in park / neutral. You are also correct that it does indeed have a Park / Neutral switch.

Thebaby

Try disconnecting the check valve at the brake booster and plugging it possible brake booster diaphragm leaking that you mask by have brake applied to put it in gear or neutral. Just a thought

trev_dl

Thanks! I will consider that. Would a way to test that be to apply the brake in park or neutral and see if it clears up before disconnecting it and plugging it?