Volvo S70 '99 Cruise control surging / pulsating, headache!

Pater · Post by **Pater** » 06 Jun 2018, 12:35

Hello guys, I'm new on this forum although I've been reading here for a while.
I'm the proud Dutch owner of a S70 '99 with 200k kilometres experience on it.

From the moment I bought the car (1 year ago) I'm experiencing some troubles...
Normally I'm not the guy who registers on a forum when running into troubles with a car, but this one is really giving me a headache!
As the local Volvo dealer is unable to help me, I decided to register on this forum.

The problem is that when cruise control is activated, the car surges / pulsates while driving 80-90 km/u (50-56 mph). The speedometer is indicating a stable speed, but the surging is definitely noticeable. Other speed ranges seems to be fine.

What I've checked / figured out so far:

- New ETM + newest software
- New MAF sensor
- MAP sensor output OK
- ICV checked and cleaned
- Front reluctor rings are replaced
- ABS wheelsensor checked (all 4 same stable speed signal)
- Vehicle speed on CAN network OK
- Swapped ABS module, gave no improvements. (Resoldered ABS module).
- Fuel pressure: steady 300KPa
- Spark plugs are all looking the same (whitish, light grey).
- No DTC's captured at all.

Some numbers:
Engine: B5254S
Motormanagment: Denso
Gearbox: M56 (manual)

During surging/pulsating on cruise control I noticed that the throttle valve fluctuates between 15 - 21% (6%).
On different speeds where no surging is noticeable the throttle valve fluctuates 1-2%.
This fluctuation is driven by the ECM as the desired throttle value is changing quickly during this surging. It seems like the ETM is just acting on this signal coming from the ECM (desired and actual throttle values are close to each other)

Car runs perfect except for the cruise control. On idle the RPM fluctuates from 835-865, I suppose this is normal? The throttle valve acts on this.

All read outs have been performed with a DiCe tool.

What sensors / signals are capable of influencing the ECM's desired throttle value?

Could someone please explain why my cruise control is behaving like this?

Thanks!

Pater · Post by **Pater** » 07 Jun 2018, 04:09

Does anyone know what values (range) the throttle valve gives at idle?

Mine is fluctuating inbetween 0,5 - 1,5 %. You can slightly hear this fluctuating at idle. Just wondering if this is normal or part of the problem....

Post by **abscate** » 07 Jun 2018, 04:30

I can check on the way home this afternoon.

Pater · Post by **Pater** » 07 Jun 2018, 04:37

That would be really be apreciated. Is your engine using a Bosch or Denso motormanagment?

Post by **abscate** » 07 Jun 2018, 18:59

Just got home. Will measure in morning , sorry

Pater · Post by **Pater** » 09 Jun 2018, 05:19

Did you had a look in the meanwhile?

Klausc · Post by **Klausc** » 11 Jun 2018, 18:31

The idle speed should not fluctuate. Check your intake manifold vacuum as you might have a very small leak. 80-90Km/h is almost idle speed in top gear.

Post by **j-dawg** » 11 Jun 2018, 23:34

I noticed recently that mine handles moderate grades at about 65mph very poorly. It seems incapable of generating a moderate amount of boost on hills - it goes straight from 0 to 4 psi or so. Then I tried it with my own foot and realized that I myself could barely moderate the throttle to keep the car at exactly the speed I wanted it on that hill. It strained my calf to keep the car climbing that exact grade at that exact speed.

Next time you notice it fluttering, cut off the cruise control and try to maintain your desired speed with your foot. You might find the same thing I was running into: at a certain engine load, around a certain throttle setpoint, the throttle mapping is extremely sensitive, and the cruise control's inputs don't have sufficient damping to handle it gracefully.

So on my car at least, it's not just speed, but speed and throttle setpoint that cause this issue. A thing you could try: next time you're going uphill or downhill, try going at the speed that gives you trouble. If it now can hold the speed, you have brought yourself out of the combination of engine speed and engine load that gave you trouble by changing only the amount of torque you were asking the engine for. If the throttle is sensitive around 3000rpm and 20% throttle on flat ground, maybe it's fine at 3000rpm and 40% when you're going uphill.

Now how to fix that issue, I couldn't tell ya. In the five years I've had the car, I've only run into that combination of speed limit and incline once. Sounds like your trouble combo is more regular for you.

Post by **scot850** » 12 Jun 2018, 07:35

Maybe it is around the speed top gear locks up at. I think that is around 90kph or slightly higher. If that is the case then maybe the engine is working hard to maintain speed in that condition?

Neil.

Post by **abscate** » 12 Jun 2018, 08:04

My T5 throttle sits at about 8-9% when started, after warmup 5-6%

There is little fluctuation, small movements over seconds. Ill post a picture of the time trace here