Intermittent High Idle - DEQ Fail

XJ-S
1

I’m having an intermittent idle issue with my 1986 3.6L 6cyl. Most of the time, the idle is fine, but after the car is warmed up, sometimes the idle is sticking high (around 1800rpm). Sometimes it drops back down after driving a few blocks, and sometimes it sticks high. As a result, it failed emissions testing because the idle stuck high on the second test.

I don’t think it’s a vacuum leak since the problem is intermittent. I removed the AAV and tested it–it’s the model that doesn’t quite close all the way, but it does seem to be working based on a hot water test. Is there anything else anyone is aware of that could be causing this issue? I saw some similar posts about this from other 6cyl owners but mostly from later cars that don’t seem to have the same parts.

2

I also have a 1984 3.6 & I believe the throttle body needs cleaning.

Best regards Jim 1984 3.6 XJSC

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3

A faulty PCV valve (spring failing) creates a loss of vacuum and over revving and these in the early stages of failure can be intermittent.

4

I didn’t think the 3.6 had a PCV. Do they?

5

Hmm…we cleaned it with some carb cleaner, but the problem persists. We didn’t take the whole thing apart, though.

6

You can block the hose to the AAV & see if the idle drops. If not, it’s
the throttle or a vacuum leak. There is a vacuum hose to the ECU in the
boot (does yours have this) & another to the distributor. They can be
checked for leaks with the hand vacuum pumps used for brake bleeding.

I believe the idle on later cars is set by the ECU & a stepper motor
More complex.

Best regards

Jim 1984 3.6 XJSC

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7

Ya gotta take the throttle body off. Wait till you see what it looks like on the other side.

It’s four 10mm bolts. It’ll look like you need a gasket at the intake, but there isn’t one.

Other possibilities:

  1. EGR valve
  2. IACV valve.

I’d still take the time to fully clean that butterfly, from both sides.

8

Cleaning the PCV valve often doesn’t do it Replacing it is the go. As said clamping off the engine end will test it.

9

You have pre 87’ version of AJ6 where the ECU is almost the same as in v12.

Three more things to add:

  1. Fuel pressure. If the issue kicks in randomly - I would start from new fuel filter and sump tank cleaning. Fuel pump on the way out may give similar symptoms.
  2. Coolant temp sensor - if not checked already, this should be the start point
  3. Air temp sensor - as per pt. 2

Now:

  • what is the mileage on the car?
  • Is your banger equipped with cold start injector ( kinda thing under the middle of the fuel rail with injector wiring plug on it). If so - disconnect it.
  • you definitely have AAV.
  • have you checked suplementary air valve for leaks? (Clamp the hose with something)
  • have you tried elimination method? (Unplug coolant sensor and check, air temp sensor and check, potentiometer and check - one thing at the time…)
  • have you tried unscrewing oil cap whan issue kicks in?
  • have you tried to manually force the throttle body to close when it revs high?
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10

20+ characters of useless garbage here so my answers will post.

11

  1. Checking the NGK sensor is the right thing to do. There is a thing with small batches of replacement parts for the vehivles no longer in production - these parts don’t like quality and manufacturing SPC. It’s like a sale in UK Primark - “everything must go”

  2. It seems you’ve missed the minor issues checks prior to going into full drama. I strongly suggest to confirm if your throttle is closing entirely. There is a return spring above accel. pedal which is getting weaker over time. Without its full support the springs on throttle body won’t be able to close entirely. You may experience it right after lubricating linkages with something thicker than oil etc.

  3. Suplementary valve check is the easiest, just put small c-clamp over the hose, or simply blank it off. It works only only on and below -10deg.C anyway

  4. Yes the cold start injector may contribute to your issue

  5. Unscrewing oil cap will introduce extra air to the inlet, the engine revs should react and accordingly.

Also - have you noticed that 1.8k revs sre kicking in randomly (with accel. pedal pressed, static at any position) or after releaseing it?

12

The stickiness is happening in neutral when I’m fully stopped (car is manual). Sometimes I’ll stop at a stop sign and the revs stick at 1.8k. Other times I’ll stop and it’ll stay nice and even at 700ish (can’t remember the exact number off the top of my head, but it holds steady).

We did clean the throttle and lubricate the mechanism and make sure the return spring was working–it seems to be.

13

In the past - I have done the same and everything was working fine until I’ve attached the linkage. The ratio of closing force was severely reduced. From the first impression it looked like dirty throttle mechanism, then it become clear after linkage disconnection.
I must admit - I am unfamiliar with manual configuration, however apart from throttle body having only one linkage attached - nothing should be different.

I strongly suggest to have a look at simple things first - you don’t need to run the engine - an assistan will be mandatory to allow you inspection of the throttle (is it going all the way back and hits the adjustable stop?)

Lubricating your cable with oil won’t hurt…

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14

Well, the car is back together. I put the original coolant temperature sensor back on. The high idle persists. There is kind of a roar that accompanies it when it gets stuck. I was able to get it to stick while out on the road today and I tried making sure the throttle was completely closed (it was; it’s definitely not sticking open, so I think it’s some kind of feedback-to-ECU issue). Today I took video of the problem, not that there is much to see under the bonnet.

Video 1: Sitting in the driveway revving the car. The first time it drops right down to normal idle. Second time it catches briefly and then drops to normal. Third time it gets stuck and you can see it/hear the roar.

Video 2: What it sounds like under the bonnet when the idle sticks high (you can really hear the roar in this one).

TPS still the most likely culprit? FWIW, I did confirm that this car has a throttle switch (the evil black one rather than the updated red one).

15

Good luck getting usable knowledge on that engine! Most of us have only read about it!

It doesn’t sound like a TPS problem to me, though. Admittedly, the “roar” might actually just be the belt-driven fan revved up; a symptom rather than a cause.

I’m betting that engine has a throttle bypass valve on it somewhere, since such things were de rigueur for meeting emissions requirements. I’m betting it needs adjustment.

16

Finally back to report that the intermittent high idle problem turned out to be caused by the mechanical advance in the distributor sticking. Hope this helps out anybody trying to troubleshoot a similar problem.

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