TDI Engine - Diagnostics 101

Turbo Steve

Top Post Dawg
Jan 11, 2000
TDI'ers, your vehicle might be under warranty right now, but sooner or later, you may end up working on it some day. Obviously, this could present you with some new challenges - there is a first time for everyone; perhaps this might get you started.

Here's a few basics that I have learned in the automotive industry that might provide some assistance in performing very basic diesel engine diagnostic procedures. Many of these procedures are the same with gas and diesel engines - many of them are not:


Repair procedures aren't the same as what you're used to in the good 'ol days, with the biggest differences occuring within modern electronic engine and transmission technology. Diagnosis of the problem is now our main concern. Many mechanics can swap out parts, but the very best ones are experts in correctly diagnosing the real problem.

When a diesel engine misfires, for instance, you know that you won't be looking for a faulty spark plug or wire (unless you're Ric
) because those parts simply aren't present.

The following overview is intended to help you get started in diesel diagnosis with confidence. Most of what I say here applies in a general way to all diesels, but I'll use the popular Volkswagen TDI engine as a specific example.

Internal Component Basics

To begin with, ascertaining the condition of a diesel engine's internal components requires some different thinking. Since these powerplants are very noisy compared to spark-ignited engines it's much more difficult to hear a bearing or wrist pin knock. Some experience and the use of a stethoscope and an accurate oil pressure gauge will help here, plus consideration of the odometer reading and the kind of care the car has had.

Bearings and pistons take a severe beating in a diesel because of high cylinder pressures, but bear in mind that they're also engineered to be much more rugged than those of gasoline engines. Another factor to take into account is if starting "ether" has ever been used. I've personally seen this practice actually snap a crankshaft, so you can imagine how much it can damage bearings, pistons, and pins.

A vacuum gauge may be useful for diagnosing gasoline engines, but since diesels are unthrottled, they naturally produce next to no vacuum, so the gauge won't tell you anything.

Compression is the Foundation of a TDI Engine

Just as in any other engine, a compression test can be used to find out how the rings, valves, and head gasket are holding up. The readings are taken at the injection nozzle holes or glow plug holes, depending on the manufacturer's recommendations. Remove all the nozzles or glow plugs before cranking so that the engine can spin as fast as possible, and disconnect the fuel shut-off solenoid wire to stop flow to the nozzles. Of course, the gauge must be capable of withstanding much higher pressures than those of gasoline powerplants.

Typical specifications are 400 to 500 psi, and if a specimen isn't up to these readings, it may be difficult or impossible to start (after all, compression is what ignites the fuel in a diesel). If one cylinder is 75 psi or more lower than its neighbors, it'll be apt to misfire, causing roughness and a loss of power and efficiency.

Once you've zeroed in on a "weak cylinder," squirt a small quantity of engine oil into the combustion chamber and do the test again. If your reading goes up dramatically, the rings are probably at fault. If the rise is small, bad valves are probably indicated.

Another distinctly possible failure is a blown head gasket, since a diesel's high compression (pressures) put terrific demands on whatever seals the joint between the block and head. A good indication that this seal has failed are very low readings on two adjacent cylinders. By the way, don't forget to check the condition of the timing belt!

Compression testing tells you the condition of an engine's foundation, and should be done when you suspect internal parts of causing a problem. But there's a lot more to diesel diagnosis, and I'll break it down into the proper procedures for specific complaints.

Reluctance of TDI Engine to Start

An engine that's hard to start or won't start at all may be suffering from several ailments. I've already mentioned lack of sufficient compression, so I'll move on to the fuel system.

The first thing to check is the supply and condition of that liquid hydrocarbon. A typical injection pump will have an internal vane pump that moves fuel from the tank to the components that generate the high pressure needed at the nozzles. And here's where you can make a costly mistake: I've seen cases where an injection pump was replaced only to find that the engine still won't start. The real problem? A vacuum leak (usually at a connection or the filter) or blockage in the supply system between the tank and the pump intake.

VW's have a clear line between the filter and the pump, and if you see a steady stream of bubbles (a few are okay), you can be pretty sure there's an air leak. A good quick test is to detach the hose from the outlet of the filter, immerse its end in a container of fuel, then crank. If the engine starts now, you can track down the leak by applying vacuum from a hand pump to the line from the filter and disconnecting and plugging various joints from the tank forward until you get the vacuum to hold.

Blocked Fuel Lines or Filter

Blockages may have several causes. Look for a crushed line or a collapsed hose, then see if the filter is clogged. Another possibility is a restriction caused by the build-up of algae or fungus that sometimes grows in the tank, especially if there's water in the fuel. Finally, if you live where it gets cold, the formation of wax crystals in the fuel can stop flow. Typical 2D has a wax appearance point of about 10 deg. F., so if the temperature is around that, look for cloudy, whitish fuel. If you find it, that's probably why the engine won't start.

The test mentioned above (substituting a supply of fuel from a container) is useful in detecting fuel contamination and flow problems, too. Also, fuel with water in it will appear to be = full of bubbles as it moves through a transparent hose.

Once you're sure fuel supply is okay, next find out if anything is getting to the injection nozzles. Loosen the gland nut that holds the line to one of the injectors and crank the engine. If nothing leaks out, there's a good chance that the fuel shut-off solenoid isn't getting juice. If the lead shows 12 volts with the key on, then the solenoid itself should be replaced.

If everything is all right so far, check the operation of the cold start advance mechanism, which makes injection occur earlier than normal to aid the engine in firing up. On the VW, this is controlled by a knob on the dash. See that the lever on the pump moves the full distance between its stops.

Injection Pump Timing

An injection pump must be properly "timed" for the TDI engine to run. On older TDI's, you'll need a special adapter and a dial indicator to check and adjust this setting on Volkswagens, while other engines require different procedures including in some cases the use of a timing instrument that "sees" the flash of combustion or senses the pressure pulse in a injector line. Look up the proper instructions in the manual for the car at hand. Again, consider the timing belt in those engines that use it to turn the injection pump.

Inoperable Glow Plugs

Especially in cold weather, a faulty glow plug system can result in a hard-start or no-start condition. The first thing to ascertain is whether or not the plugs are getting current. Temporarily detach the wire from the engine temperature sensor so that the system will operate regardless of how warm the coolant is, connect a test light to a good ground, turn on the ignition key and immediately probe the glow plug connectors with the test light. You should find voltage for a specified amount of time, then your light should go out (remember to turn off the key). If there's no current reaching the glow plugs, check the fuse first, which may be in-line or in the regular fuse panel. If the fuse is okay, check the wiring and any fusible links that may be in the circuit. If you still haven't found the problem, the glow plug relay is probably the culprit.

In cases where the glow plugs are receiving voltage, but you still suspect that the system is the source of the starting problem, see that none of the plugs is burned out (electrically open) by removing the wires or bus from all the plug terminals, connecting the lead of your test light to the positive battery terminal, and probing each plug's terminal. If the light doesn't come on at one glow plug, that plug no longer has continuity to ground and should be replaced.

An ammeter in series between the feed wire and the bus can also be used to check the plugs. On a Volkswagen, you should see an initial draw of 140 amps, which should then drop to 36 amps. If you get a reading significantly less than that, a burned-out plug is indicated (for example, one bad plug will result in a 27 amp reading). You can test the plugs individually, too - replacing any that draw more than 9 amps or draw no current at all.

Even though the glow plugs may check out electrically, they might be unable to do their job because of heavy carbon deposits, so remove and clean them as necessary.

Roughness of Engine

Rough idling and missing are, of course, indications of imbalance in the engine. Besides compression problems, poor injection to one or more cylinders is a common cause. The initial test is similar to the time-honored practice of pulling spark plug wires one at a time on a gasoline engine to find the one that doesn't affect idle quality. With a diesel, you disable the cylinders by loosening each fuel line at its nozzle so that leakage relieves enough pressure to prevent the nozzle from opening (wear goggles and be careful to direct the escaping fuel from possible sources of ignition - use a piece of sheet metal or cardboard as a deflector). If disabling one cylinder has little or no effect on how the engine idles, that's the one to concentrate on.

Indications of Weakness in a TDI Engine

But there's also an interesting indirect means of finding a weak cylinder: Measure the electrical resistance of the glow plugs, which can be done easily using a low-reading digital ohmmeter. With the engine idling, disconnect the wires from the glow plug terminals and record the resistance between each plug's terminal and ground. The hotter combustion makes a plug, the more resistance its element will have, so a cylinder that's operating inefficiently will show a lower ohm reading than one that's working well. Unfortunately, other factors can affect the readings, so this test won't always produce accurate results. It's still a useful technique, however, provided you don't rely on it entirely.

Yet another means of locating a weak cylinder is to measure the temperature of each exhaust manifold runner, for which you'll need a very accurate digital pyrometer. Any cylinder that produces a low reading should be suspect. As with the resistance test described above, this is just a helpful indicator, not a definitive test.

Timing Belt

Low compression in several cylinders could also mean that the timing belt is broken and possible engine damage has occured!

Injector Nozzle Check of Problem Cylinder

Once you've isolated a problem cylinder, remove its nozzle and examine it for obvious damage and deposits. If you have a "pop" tester, attach the nozzle to it and apply pressure (be careful not to place your free hand under the injector as the high pressure spray can penetrate skin and cause blood poisoning). You should see a compact, even, cone-shaped spray pattern when you operate the tester with rapid strokes, and hear a creaking sound with slow strokes. Check the opening pressure against specifications. On Volkswagens, this should be 1,700 to 1,850 psi, and the nozzle shouldn't drip when pressure is held for 15 seconds.

Any nozzle that has a poor spray pattern or an opening pressure significantly different from specifications or from that of its companions in the engine should be replaced or overhauled. The Robert Bosch units found in the VW and many other diesels can be disassembled for cleaning, and the shim above the needle spring can be changed to alter opening pressure (the thicker the shim, the higher the opening pressure).

Injection Pump

Diesel injection pumps are extremely well-made and normally last almost forever providing they don't get a dose of contaminated fuel. They're also supremely expensive. Unless every other component checks out okay, and/or you suspect that bad fuel is present, the pump is probably not the cause of the problem. In any case, be very sure of your diagnosis before you condemn an injection pump, and, unless you intend to become a specialist and to invest a great deal of money in special equipment, don't try to service them yourself. Replacement with a new or rebuilt unit is the general rule.

For TDI'ers having problems beyond their basic level of expertise, it would be wise to find the best diagnostic mechanic available, even if the labor rate is higher. In doing so, you'll actually save money in the long run, by simply having the problem diagnosed properly and repaired correctly the first time!

[This message has been edited by Turbo Steve (edited February 11, 2000).]


Wow, that is awesome, a very good read at that. I have one thing to add to your great review.

On the new A4 TDI engines, the oil is cooled by the radiator fluid at the base of the oil filter. This is a good setup to maintain cool clean oil (you will notice the oil supply for the Garrett VNT15 turbo comes off here right at the base of the oil filter, insuring the coolest cleanest freshest oil goes right to the turbo bearings first!), however it is also a weak link in the overall oil supply system that could lead to either "oil in radiator fluid" or 'radiator fluid in oil" leaks. The first is easy to spot as the coolant reservoir is clear and oil sludge scum buildup is readily apparent to the naked eye (if "naked" eyes offend you, simply wear sunglasses
). If the radiator fluid is leaking into the oil supply it will also become apparent as the radiator fluid drops and ethylene glycol-water mix is much more damaging, unfortunately,to the engine when it ends up in the crankcase. Therefore, excesive white smoke at startup or under operation out the exhaust should be investigated to see if the radiator fluid level is still normal. A leaking head gasket could also be the cause of this problem.

Also, the VW oil filter system is has an "oil filter by-pass valve" that shunts oil past the oil filter when the oil filter is clogged up and not working, most likey from overextending it's normal change out or forgetting about it all together. On cold startup, the VW TDI oil pressure can easily exceed 100+ PSI and the bypass valve then shunts unfiltered oil to the engine. This is not all that bad in itself compared to no oil pressure or delivery what so ever. However it is extremely detrimental to your TDI to have a dirty over extended oil filter since when the bypass valve opens, it blasts all the collective crud and dirt which it has collected for thousands of miles out directly into your engine!
This is bad, real bad and is one of the major reasong to adhere to routine regular oil filter changes instead of forgetting about it or attempting to over extend your oil filter mileage.