Surging/hunting while slowing down

**Konradical** · 18th July 2019, 08:46 PM

I just want to get others thoughts on this; when slowing down from say above 60kmh, but below 90kmh, my 89 does this weird surge or hunt. Does it with or without my foot on the brake and if I hold it in a gear below drive, it won't do it, even at those speeds.

It started doing it some time ago, but either it went away or I got used to it and since I have had to drive it everyday again this week, I have noticed it happening again.

Possibly transmission related? Or a vacuum leak?

3.5lt flapper with zf auto.

Cheers

**FisherX** · 19th July 2019, 08:01 AM

I'm thinking vacuum leak. But I'd say a decent size one.

I use a EVAP smoke machine to find leaks that I got off eBay for about $100 bucks. Just like the one Scotty Kilmer is using in the youtube link.

YouTube

**Konradical** · 19th July 2019, 04:35 PM

Yeah vacuum leak was my first thought. I had in the past right through all of the lines and replaced where needed, as well as doing the ol aerostart test.

I did think it could be the brake booster, but it doesn't do it at idle and it does it without my foot on the brake.

Having a bit of a play/test on the way home this afternoon, I found that it does do it in other gears. Unfortunately my taco isn't working right now, but it seems to be at about 1000-1500rpm and goes away just above a neutral idle. Kind of sounds like the engine is labouring at the point in the rev range against the gear.

Maybe I need to adjust the kickdown cable..

Does anyone have a proper adjustment procedure for that, the adjustment, that doesn't involve driving around.

**PhilipA** · 19th July 2019, 07:20 PM

If it has a 14CUX which some 89s had with a 3.5,the injectors cut off at over 1500RPM and closed throttle.
The injectors restart once under 1500RPM for a smooth change to 1st and the Stepper motor holds revs over 1100 for the same reason.
I don't know if the earlier flapper does the same.
If it is a 14CUX it is normal, but once you notice it you become sensitized.
Regards Philip A

**Konradical** · 19th July 2019, 10:20 PM

Originally Posted by PhilipA

If it has a 14CUX which some 89s had with a 3.5,the injectors cut off at over 1500RPM and closed throttle.
The injectors restart once under 1500RPM for a smooth change to 1st and the Stepper motor holds revs over 1100 for the same reason.
I don't know if the earlier flapper does the same.
If it is a 14CUX it is normal, but once you notice it you become sensitized.
Regards Philip A

Thanks Phillip, it is def the 4cu version with the flapper. I understand the version you are talking about only came in the discovery.

You have given me an idea for something else to check though, they have a over run relay which cuts the ignition pulse to the ECU when the engine is under high vacuum situations, which would include slowing down.

Thanks again.

**PhilipA** · 20th July 2019, 07:55 AM

I understand the version you are talking about only came in the discovery.

No it was in all 3.9 including RRC and Disco from 89 onwards to model end in 1994.
Regards Philip A
Whoops including all 3.5 Discos and Disco1 to model end in 1998.

**Konradical** · 20th July 2019, 08:04 AM

Originally Posted by PhilipA

No it was in all 3.9 including RRC and Disco from 89 onwards to model end in 1994.
Regards Philip A
Whoops including all 3.5 Discos and Disco1 to model end in 1998.

Yeah, sorry, I was referring to the 3.5lt, not the system in general.

The EFI 3.5lt RRC is a bit special!

**Mercguy** · 24th July 2019, 01:14 PM

Originally Posted by Konradical

Yeah, sorry, I was referring to the 3.5lt, not the system in general.

The EFI 3.5lt RRC is a bit special!

the RRC period, is a bit "special"

Doesn't mean we don't care, it's simply the truth.

**Konradical** · 24th July 2019, 01:16 PM

Originally Posted by Mercguy

the RRC period, is a bit "special"

Doesn't mean we don't care, it's simply the truth.

Ha! Very true!

In turn, it probably makes us special..

**Mercguy** · 24th July 2019, 01:25 PM

Just as an aside, and for 14CUX 3.9 owners out there, it is advisable to ensure the PCV system is functioning freely and has no blockages or restrictions.

when the flame trap, hoses, restrictor valve t piece and the IACV solenoid have contamination, the condition of high or erratic idle is prevalent.

A routine cleaning of the flame trap, hoses and the stepper solenoid / IACV will ensure the smoothest transition possible.

remove and clean at every oil change, use a foaming cleaner like threebond engine conditioner / nulon/subaru upper engine intake cleaner and blow them out thoroughly before refitting. the back of the intake plenum where the solenoid / iacv is located should also be sprayed and cleaned out properly.

This is a prevalent issue on modern NEW cars as well. It's not restricted to old vehicles. Modern EGR and PCV systems are actually much worse, due to the emissions regulations and the methods used by manufacturers to meet those regulations in the most cost-efficient manner....

Most dealer servicing will completely ignore this and not clean any of the auxillary air intake system (EGR/PCV/AAV) and eventually the intake ends up clogged with carbon and crap like you wouldn't believe.

Direct injection engines suffer more, because the back of the inlet valve and the inlet port are not being sprayed with atomized fuel, which acts like a cleaner through the induction process.

blah blah blah etc etc etc... just check it and clean at each service and the preventative maintenance will go a long way to keeping the intakes clean.

always read the instructions on the can and do not use the can in a diesel engine, unless it says it is safe to do so.... runaway diesels are not fun.

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