Vehicle details: Smart ForTwo
Year: 2004
Symptom: Engine misfire,
Engine stalling
Author: Matt Williams | Pico
Products suited to this case study*
  • WPS500X Pressure Transducer Kit (with carry case)

  • WPS500X Pressure Transducer

  • *At Pico we are always looking to improve our products. The tools used in this case study may have been superseded and the products above are our latest versions used to diagnose the fault documented in this case study.

Smart ForTwo | Misfire

This is a real-life diagnostic article with a slight difference from the norm. This story concerns an employee of Pico Technology and their journey to the office. About one mile from work the owner of this 2004 model year 54,000 mile Smart Fortwo started to experience a very noticeable misfire. This seemed like an ideal opportunity to prove the capability of the automotive kit!

The Smart Fortwo is a rear-engined two-seater car manufactured by Smart GmbH and introduced at the 1998 Paris Motor Show. The Fortwo employs a relatively rare, small-capacity 3-cylinder turbocharged engine.

With the vehicle displaying no warning light and without any codes available via the serial tool, we first decided to take a best guess approach and check the ignition system.

This displayed no noticeable ignition system issues across all cylinders, but it did display some irregularities on one cylinder during the burn phase: could this be air turbulence within the cylinder?

The next step, based on the ignition information gathered and driven with a little knowledge of the engine, was to carry out a compression test. To do this we used PicoDiagnostics (available to download free of charge alongside PicoScope 6 Automotive), in order to conduct a relative compression test.

This is simply done by connecting a standard test lead across the battery terminals. The vehicle then needs to be immobilised to prevent it from starting.

The software will then ask you to crank the engine for roughly five seconds while it collects enough data for the calculations. The results from the Smart are shown below:

As we can see, one of the cylinders is extremely low! The software will place the highest-value cylinder at the leftmost and label it as 100% with the remaining cylinders displayed alongside.

The next step was to connect the WPS500 pressure sensor in place of one of the spark plugs. The test is then carried out in exactly the same way except that, this time, numerical values for pressure are assigned to the cylinders.

This is done by taking an actual value from the WPS500 pressure sensor and therefore identifying the cylinder that is connected. Then, using the relative measurements taken through the battery connection, the remaining cylinder values are calculated and displayed in firing order.

The results are shown below. Please note that the inclusion of the pressure sensor and its coupling hose has caused some variation from the previous measurement, but this can be allowed for within the configuration of the software.

This was now enough information to suspect mechanical issues within one particular cylinder. The next step involved more traditional tools!

As the picture clearly shows, the exhaust valve has sustained some serious damage!


18 comments | Add comment

April 03 2014

This is such an awesome tool,i will get one soon.This is so cool how it does all these different tests in a very short time.Thanks PICO.

August 23 2012

Interesting, but it just goes to show how little people know about Smarts, you have diagnosed the symptom (burnt valve), but the cause will actually be sticking piston rings and therefore loss of oil control on cylinder walls! Change only the valve and you will do it again next month too.

December 31 2010

This exemple shows different use of the scope and its accessories. In our workshop, on audible suspicion of a misfire, we will go first for the easy and quick relative compression (before looking at ignition or VE), with a pick up on cyl #1. Then check the suspected faulty cylinder with the wps500…the full compression cycle will tell the complete story. Roughly 5 mn, one technician, to fully diagnose the issue with accuracy and minimal activity. I bet my old foreman will also go for a smoke test to see the leak (he doesn’t like PC). It requires a much longer time to prepare the quote…
Then it is certainly a good practice to redo the wps500 compression test after the fix, to make sure there isn’t another hidden issue/cause and demonstrate the job performed to the customer (ours appreciate the before/after).

Aaron Firth
June 15 2010

Got to agree with how quick the relative compression tester is, whats quicker than just clamping on a battery?

Samuel Jarrvis
January 07 2010

I am somewhat surprised the Pico Diagnostic cylinder contribution software was not run first.  I have had mixed results with the cylinder contribution tests. So I can understand.

In my experience the relative compression test has been very accurate with one exception.  A bad starter one time resulted in an inaccurate test result.

Something I believe most people do not realize is the Pico Diagnostic relative compression test uses only the battery connection.  Most times a high current inductive amp probe is required. This I believe is due to the high resolution of the 3000 and 4000 series scopes.  It is possible to record DC voltage then zoom it times 50 and see the swings as if it were AC coupled.

A couple of thoughts on the captures. It is possible to utilize the second channel to display other measurements.  Since this was a bit of play and test it would have been interesting to show either the first look or a pulse type sensor displaying that on ch B and showing the graph or jump the WPS to ch B and display the compression waveforms.  Even a current probe on ch B could have been cool.

While a mechanical compression gauge has its place there is a lot that can be seen using compression waveforms and intake and pressure pulses.

Overall I find the relative compression test my most used Pico Diagnostic (software).  It is super quick to use with one connection to the battery for a quick go/ no go on engine mechanical.  Sure if I hear an engine cranking unevenly I will dig further for a mechanical issue.  It also builds value for me and the shop.  I can print a professional looking report showing test results.

Samuel Jarvis

Dave F
November 25 2009


The design of modern engines is changing, I suppose in a way catching up with the boom in electronics. 
We have multi valve, VVT, variable valve lift, variable intake etc which all affect engine breathing. As others have said, access to spark plugs on some of these units to do what were once quick tests can present quite a challenge.  The use of pressure transducers which can monitor mechanical events ( intake, compression and exhaust ) can offer a great deal of non invasive diagnostic information in a short space of time. 

I didn’t read this article and think about how much quicker I could have done it with a compression gauge, I was thinking about how investing in methods and equipment like this will improve efficiency and accuracy on the more complicated engines to come. This example by Pico is only a simple demonstration of one basic application for their pressure transducer.
For me it’s about learning on the simple stuff then you know what you’re looking at when it matters !

Just my thoughts

Dave Foye

November 24 2009

Yes i think this test has its place as jez has said on a V6 Freelander the rear spark plugs are under the variable intake part of the manifold, & this would be a far more cost effective & quick test.

November 23 2009

Imagine this was a V6 or something! far quicker using the Pico than a compression tester. I reckon you could diagnose a major engine fault within 5 minutes, how long would it take to remove each plug etc etc?

Matt Williams
November 21 2009

Firstly I would like to thank all for taking the time to leave a comment on this article.

Unfortunately I think a few are missing the intended point, that being to show a usage of our product.  It is of course true to say the issues with this particular vehicle could have been diagnosed via other means.  However I am sure some have realised that in using our equipment the problem can be recorded, thus evidence can be given to prove the need for a rebuild.

Also good news for those interested in the pressure sensor it is now available!

November 20 2009

Far to complicated simple method remove plug realise something wrong compression test

November 04 2009

Sure, tease me with the pressure transducer that I cannot yet purchse. LOL !!

Les Miles
October 26 2009

You may have fun playing around with your own car but for a customer I would have done several quick things. 1 listen at the exhaust(so simple) 2.compression gauge would have told all the story 3.needle bounce at the manifold suction test.
Here I say have fun if you have nothing better to do by all means but not at the exspense of the poor motorist—go and refresh your basics

October 26 2009

Thanks for the teaching on this, i think you could find this problem with basic tools faster and easier like a vacuum gauge then spot the bad cylinder. The burn valve was because of the valve not seating because of excessive carbon built in the valve seat or exhaust leak.

October 24 2009

Ah, in the old days you would have run the engine and removed and then replaced each plug cap in turn. This would have identified the cylinder. Removed the spark plug to check for good spark and found the spark plug burnt indicating too lean a burn. Removed plug leads from other cylinders, put thumb over plug hole and brief crank of engine would have found the lack of compression. I am not knocking an excellent product or manufacturer, I use their products daily but if a motor mechanic needs to go to this length then he/she will definitely miss the cause of the problem and simply replace parts and reassemble.
As to the cause, thats where Pico do come into their own.

October 24 2009

To diagnose a failure to bring equipment worth thousands of dollars is not necessary. Suffice kompresometr for $ 40

October 24 2009

Although I love my Pico wouldn’t a compression gauge found the same fault? Its time and money while I happen to have a Pico scope lets always complete the testing with the fastest and cheapest equipment/ For us and the customer. Driving a Smart is all about keeping the cost of ownership down.  Thank for a great article I did enjoy it especially since I also own a Smart.

October 23 2009

Although not in the motor industry, I follow these articles with great interest.  This particular one would benefit from more detail.  How was the ignition system checked, how does the cranking current derived pressure test work, and how about some captions on the photos.  Great series though, love them.  [Also makes me wonder about reliability of highly stressed small engines.]

Dave Baxter
October 23 2009

And that “suddenly” happened?

I’d suspect something else caused that valve to burn, especialy if it did indeed do all that damage in only “one mile”...

The battery voltage/cranking current compression test, is a new twist on an old exprienced machanics ear.  Likewise, I suspect the engine exhibited symptoms an experianced ear (or sympathetic drivers ear) would have detected, prior to the terminal valve burn.  Was the valve seat damaged in any way?  How about the fuel injector for that cylinder?

All good fun.


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Case study: Misfire