I believed I had solved this issue after a new alternator replacement (for a different reason) however I am still having an unusual load or engine issue. It happens daily but not constantly. It happens while in park, it happens in neutral and it is worse in drive with brakes applied. Sometimes the engine runs perfectly smooth and quiet and sometimes it is quite rough and gets worse with each added electrical load introduced such as windscreen heater, rear screen heater, seat heaters, suspension compressor, AC compressor engagement, headlamps, steering gear usage, etc, there more, the worse it gets, enough so to be felt by all occupants, rattles interior bits and plastics, etc and can be heard as a rumble in the exhaust. Sometimes it is rhythmic and can be timed to come and go every 5 seconds and I've monitored this timing every time it has occurred and the tach needle rises and falls in time with it...touch the throttle and it goes away, release the throttle and it returns. Three main dealers have not been able to solve this in over 2 years of trying (not that it says a whole lot about it).

Anyone have thoughts?

 

Do you have an IID tool? If so can you use live values/monitor the voltage to see what changes when you increase the electrical load? Does it start fine every time i.e. no indication that it may not be charging properly? I read you've replaced the alternator - why? What was the fault you were chasing there?

 

I have SDD. Alternator needed replacing due to erratic voltage fluctuations and numerous CAN bus problems. To be clear the shuddering is there regardless of electric load, but it does worsen or increase with each additional electrical load and it worsens with any drop in RPM. Monitoring of voltage has not yielded anything conclusive.

Injectors have been tested and inspected, engine has been apart several times including multiple gasket replacements, vacuum tests done, new spark plugs, new belts, new water pump, intake filters replaced, etc.

And while it happens daily, again it is intermittent meaning sometimes the engine is as smooth and quiet as can be even with multiple electrical loads.

This has been going on for a couple of years though I feel it is getting worse perhaps.

 

Lot of things could cause these kinds of symptoms. Which car is this (V8 or V6), and how many miles? Are you showing any trouble codes? What about fuel trim numbers? Fuel rail pressure?

First thing that comes to my mind is a vacuum leak - especially a torn PCV which would cause a low surging rough idle - but you mention that vacuum leak tests were done which should have picked up on something like that. Still, if you have the V8, it’s easy enough to get to that it might be worth checking the PCV. Could also be fuel injectors.

Not sure about the electrical load - might be a red herring. But I’d check every item on the belt drive to make sure you don’t have something going on there. The idler pulleys often go bad and can cause a vibration.

 

Apologies, hadn't realized my signature wasn't showing. It s a 2015 SC V6 petrol now approx 98K miles done. No codes. Trims vary, don't recall rail pressure but easy enough to check. Injectors were tested and checked. The intermittent nature of this problem is a challenge or like the electrical assumption, could also be a red herring...

 

Not sure your proficiency level with this stuff - so I apologize if I’m telling you things you already know (I’m no master mechanic by any stretch, but just trying to brainstorm common problems I’ve come to know about).

Short term fuel trim will vary as you drive, but overall fuel trim shouldn’t vary much just sitting at idle. Your total fuel trim (short term + long term) should not be more than about +/- 10%, and should be steady at idle. So, at idle, if your long term fuel trim is say +15% and your short term is -7% (together = 8%), probably okay. But if the total added up is more than 10%, your car is struggling to keep the fuel mixture where it needs to be. If it gets up to about +/-20% or so, you’ll start throwing codes. Positive fuel trim means running too lean (car adding fuel to make up for too much air). Negative fuel trim means running too rich (car trying to reduce fuel in mixture).

If you are over 10% fuel trim on either bank, then note whether its on both banks or only on one. Also, test at higher rpms (maybe 3K or so in park and neutral) to see if the combined fuel trims trend towards zero, or maybe get higher (or lower). Both banks - usually something like a maf sensor, vacuum leak (e.g., PCV valve) or fuel pressure in both rails (e.g., low pressure fuel pump). One bank - something that is affecting only one side of the engine, like an o2 sensor, catalytic converter, fuel injector, one of the HP fuel pumps.

If fuel trims are normal, then you can probably rule out air and fuel sources as problems. Might take a peak into your throttle body just to make sure it’s not all gummed up or carbonized.

Next easiest thing to check is probably the pulleys. The tensioner pulley’s can wear out and lead to problems. The pulleys themselves can vibrate, but they also will cause the belts to slip, causing alternator and charging system problems. The belts pull off pretty easy, and you can just check the pulleys for smooth rolling and no free play. Check the ac compressor as well - you can try turning by hand, but also just listen to it with engine running (belts on) while someone toggles the ac button.

The supercharger snout coupler also fails in these cars. Usually just causes a rattling, clacking noise, but I suppose if it got bad enough it could cause vibration. Easy check - just loosen that supercharger belt and feel for play in the pulley. There should only be about 5 degrees or less of free spin before you feel it catch and spin the rotors in the charger. It’s spring loaded, so it can have some spring play, but should not have much totally free spin.

Ignition system - not sure if you have new plugs, or if the ignition system (coil packs) were checked. That might be the next thing to look at.

The intermittent nature is important, but it could be a weakness that is simply made more apparent when something like the ac compressor or alternator starts putting more pressure on the motor. Do you notice it more when the engine is cold or hot, or more or less after a long drive?

 

Let's stick with fuel at the moment as I believe all else has been serviced already. First, fuel trims on bank 1 & 2 are close enough that I doubt any disproportion there. Second, long term trims stay positive at all times and vary very little under all conditions. Short term however, whether in park or drive and stopped are all over the place anywhere from -25 to to +27.

Throttle body is clean, super charger snout is in spec, plugs have been changed, accessory pulleys looked fine when I replaced the alternator, no codes and the issue does not appear to be temperature dependent, either ambient or engine.

 

How positive are the long term fuel trims?

 

Check your fuel rail pressure as well. You can pull up desired pressure and actual pressure on the monitor (at least on my GAP tool I can), and they should be very close to the same and steady at idle. See what they do with higher RPM as well. This will be measuring primarily the HP pump performance, which are on the passenger side of the engine. Unfortunately, there is no sensor that I know of to check the low pressure fuel pump in the fuel tank. There’s probably a spot for a manual gauge somewhere under the hood, but I don’t know where.

 

I ran the low and high pressure routines in SDD and they passed. Long term trims mostly stick to 4-6%.