Fighting technology with science

Frank goes all-out looking for the cause of a vibration on an otherwise very well maintained car

Published:  13 February, 2018

I am sure all diagnostic technicians out there will agree vehicles are becoming ever more difficult to diagnose. Two obvious reasons include the increase in networked systems, and difficult accessibility.

The first step is to conduct a non-intrusive serial evaluation. This method often provides insufficient information to progress directly to a repair solution. What if the problem is a non-monitored component, or possibly a non-monitored component causing a negative reaction in a monitored component? Sounds confusing, then you will appreciate the following diagnosis and repair review.
Here is a conundrum: What has a vibration at around 100hz got to do with a EGR fault?  

The vehicle in question is a 1.4 16v mk4 Golf 1J chassis. The vehicle history is very well known to us as it was owned by our staff member, Annette. She had it well maintained for many years despite its 125,000 miles.

It had a minor serial error relating to EGR flow. A new OE valve was fitted many years ago without success. The vehicle performed extremely well so we ignored it. The vehicle passed into my ownership several weeks ago. My intention was to prepare it for my partner’s two sons as their first car. Totally new OE brakes front and rear, four new Goodyear 185/65/14 tyres… anyone spotted an anomaly yet?

Rear wheel bearings re-packed with grease, all fluids replaced. New OE exhaust system. The car drives superbly. Brake balance differential 1%! Perfect emissions. I decided to use the car for the Pico NVH-WPS course held during a weekend in November. On the Saturday we conducted several tests to confirm the mechanical efficiency of the engine.

The primary test, following a battery status and health check, was a relative compression test conducted in the Pico diagnostics platform. It’s very quick with only the battery connected to channel 1.

The result was excellent, all cylinders returning a differential of 100%. Let’s digest this for a moment, this does not confirm good compression or correct valve timing. It’s simply a balance of voltage drop whilst cranking the engine. You know what, a bad result here always indicates a serious internal engine problem.

Testing
We then discussed the issue of pumping losses and how this can be addressed with throttle control, variable valve timing and lift, and not forgetting cylinder cancellation! This progressed to dynamic compression tests on the engine using WPS. The results were excellent showing good pressure differential (note I don’t call it vacuum as there is no such thing) suggesting efficient cylinder and
valve seal.

The day ended with a prep talk on the advantages of noise and vibration monitoring. Sunday began discussing the information required for manual data entry into NVH platform. This includes PIDs, notably engine speed via a Mongoose serial interface. All the gearbox and differential ratios were entered together with the tyre sizes. Did you spot the anomaly yet?

Basically, the software can now calculate frequency and speed against noise and vibration signatures across all engine, gear selection, and wheel speeds. Remember frequency HZ x 60 = RPM.

RPM div 60 = HZ. Down the road we went several times sticking weights everywhere to demonstrate different vibration signatures. Due to the quality tyres and general smoothness of the car there was very little vibration to look at.

However, on closer inspection there was a vibration concern around 100 HZ. Apply the maths and you get 6,000 RPM. The engine E1 was around 50HZ! 3,000RPM and there was a E2 vibration, so whatever it was had to be  engine  ancillary related. Further inspection using a roaming microphone to pin point the noise confirmed a very noisy serpentine belt idle pulley bearing. This is where the shock on my part and the realisation of the incredible value of applying science and physics to an everyday problem pays off. I decided to conduct the repair myself the next day, stripping the front end exposed a fractured timing belt guide and badly impregnated timing belt tension pulley. The broken half of the guide was hovering inside the timing cover I guess just waiting to do its worst!

Pic pulleys
Several pulleys were singing like canaries despite no previous and obvious audible noises. So, three hours later and a total front end rebuild with OE parts, including water pump, we have an even sweeter engine. So, what else did I find? My original training was as a precision engineer specifically in engine remanufacture so instinctively I don’t strip out timing assemblies until I have checked the original position. It was one tooth out on the crankshaft!

Humming, I think timing out, manifold pressure will change, it’s a MAP sensed load system, so EGR is calculated from an algorithm based on throttle, map value and EGR control ratio, with feedback.

Eventually we discover the historical problem of a seemingly innocuous EGR DTC. In conclusion by recording vibration from the driver’s seat frame, yes, I do mean from inside the car,
we pin point a potentially engine critical fault.

A mechanical non-monitored component affecting a monitored sensor value! One last thought – the anomaly! The standard tyre specification for a Golf 1.4 1J IS 185/80/14. I deliberately wanted more responsive high-end tyres. The speedo is almost 10mph out, not a bad idea for two 24/25-year olds.

Want to know more?
If you want to get on the NVH bandwagon, email Annette @ads-global.co.uk or call 01772 201597.

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