Emissions Tune up of LH Fuel Systems

Throttle bodyOld distributor cap
Winter 2004 Tech Session
Yet another great Winter Technical Session has come and gone. Like most, this one was well attended, informative, and most of all, very practical. We have Volvo of North Vancouver to thank for hosting this year's technical session. Here are system trouble codes that go along with this article. This page also features a quick summary of the fuel and ignition systems. Photos from this Technical Session can be found here. Approximately 400 kB. The distinct clicking sound of the throttle position sensor found here: 151 kB, 22 kHz, 8 bit wave file.
Readers may also be interested in the Volvo Club of BC 2001 Technical Sesssion, hosted by Don Docksteader Motors and presented by Jason Leber, Volvo Master Technician.

Wednesday, November 24, 2004 saw the 2004 version of the Volvo Club of BC ever popular and successful winter technical session. The people at Volvo of North Vancouver graciously opened up their shop bay doors to host a very informative and entertaining evening. Anyone who drives an older car in the greater Vancouver area dreads the annual AirCare inspection. Master technician Ian Peterkin took the enthusiastic audience through the tune up on a Volvo with LH2.4 fuel injection.

This article covers primarily the maintenance of the fuel and ignition systems on this era of Volvo (89 to 95 200, 700, and 900 Series) and contains some other facts on the routine maintenance of other systems. Many of the tips that we learned could well be applied to many other cars, not just Volvos. Personally, I have always been far too timid to dig into the fuel system on our 740 Turbo, but this will now change.

System History and Background

The Bosch LH fuel system makes use of a hot wire, inserted in the intake air stream, in order to meter the rate of intake air flow. This is an adaptive system, exhaust gases are monitored by the oxygen sensor, looking for the ideal “Lambda” air/fuel ratio of 14.7:1. The earliest variant of LH used by Volvo was version LH2.2. With LH2.2, timing and mixture could both be adjusted. Starting in 1989 (1990 for Turbo models) LH2.4 was standard on Volvos in Canada. The LH2.4 saw the introduction of a diagnostic user interface, enabling service personnel to very easily extract fault codes from the engine management system. Nothing is “adjustable” on LH2.4.

A description of the components of the fuel and electrical system, as well as the use of the engine diagnostic user interface, are located on a separate page. Typical fuel systems are made up of numerous sensors and a handful of injectors tied to a very specialised computer system. As long as most of the system is intact, you are virtually guaranteed to get through the emissions testing. There are a number of procedures that are scheduled service items. Prior to AirCare is not a bad time to execute these routines.


There are a number of specialised tools recommended for maintenance of the ignition and fuel management system. These items are as follows. An electrical multimeter capable of measuring voltage and resistance in the least and possibly current, too. A non-contact thermometer (frequently referred to as an optical pyrometer) for measuring surface temperature at inlet and outlet of catalytic converter. Ignition lead pulling pliers. Hose clamping pliers for cutting off air flow to the IAC. For LH2.2 equipped cars from 85-88 (1989 for Turbo models) one can purchase, from Volvo (part number 9995280), a test diode for about $50. This very valuable tool is connected to the engine management system and allows adjustment the CO. More information on how this is used can be found on the BrickBoard. And, apologies, we had this link wrong until late July 2005.

General Maintenance

Ian started out by making some comments on maintenance in general. The worst things to neglect on your car are: leave the oil too long between changes and not service the PCV system. Volvo recommends 12000 km for the oil; Ian recommends 6000 km. The PCV system should be serviced every 12000 km and the coolant changed every 36000 km. Other general maintenance items: change the power steering rack fluid, transmission fluid, and brake fluid at least as often as Volvo recommends.

It is wise to always change the engine oil immediately before emissions testing. The oil acts as a type of buffer for hydrocarbons, absorbing when fresh and then later releasing them. Oil rich in excess HC can result in high readings during the testing process.

Crankcase Ventilation

The crankcase ventilation system consists of a breather box (a hand-sized molded black plastic housing lurking under the intake manifold), a flame trap and a number of hoses. The breather box (Volvo part number 3501160) is responsible for allowing fumes from the crankcase to escape without having oil escape with it. If the breather box, flame trap or supporting hoses get plugged, the crankcase pressure will build up and will cause most if not all of the various oil seals to leak. In extreme cases, the seals may be damaged, requiring replacement. The flame trap and the hoses associated with it should be inspected to see if they are trapped with carbon. If the hoses are brittle they are getting plugged with carbon and should be replaced.

The flame trap sits on top of the breather box. The breather box cannot be successfully cleaned; it is simply replaced. The exact service interval depends on many factors.


The MAF sensor is one of the most critical components in the fuel system. A faulty sensor or poor electrical connection to it can result in a number of driveability issues. With age, moisture and exposure to contaminants, the electrical connector contacts can corrode and produce inadequate electrical connection. Servicing this connection is relatively simple. Start by removing the spring clip on the connector and disconnecting it from the MAF. Inspect the electrical contacts in both the MAF socket and the wiring harness plug. Any corrosion should be removed and the contacts cleaned with electrical cleaner. The connector pins may be loose and can be re-tensioned by the use of a small tool. Keep this connector clean and use a dielectric grease. Volvo can supply you with a green dielectric grease, available from Volvo or other auto parts suppliers, which can be applied liberally to the connector.

The mass air flow sensor is an expensive component and can be ruined if the air box thermostat is faulty. This is covered in the air-box section.

For proper operation of the system, it is imperative that all air entering the engine pass through the MAF and that all of this metered air is delivered to the engine. Vacuum leaks can throw off the adaptive control. Lean mixture may be caused by a leak in the intake manifold gasket, resulting in a lumpy idle, possibly with lean misfire. Using a water spray pump, water can be sprayed all around the intake manifold gasket and injector seals. If there is any leakage in this area, you should hear a notable sucking sound.

Air Filter Box and Thermostatically Controlled Mixing Valve

The air box is the housing for the air filter and a thermostatically controlled mixing valve. Air is supplied to the air box from fresh ambient air and from preheated air from a heat exchanger on the exhaust manifold. The air box thermostat changes the mixing percentage of the two sources to maintain a consistent intake air temperature. Unfortunately if the thermostat fails (and they regularly do) they normally fail to the hot side admitting 100% heated air. Depending on the conditions, the air presented to the MAF can reach exceptionally high temperatures. The elevated temperature can cause the MAF to fail resulting in a very expensive repair bill. Check the thermostat by removing the cover of the air box and the air filter when the car engine is warm and check that the thermostat has closed off the hot air port. If not, the thermostat must be replaced soon. Some club members suggested that the hot air feature could just be eliminated in cars in our moderate coastal climate and do away with the problem.

Air filter service should also be performed. The filter in this car (cough, gag) really needed some work. Replace the air filter if it is dirty. One club member suggests using a cleanable filter element by K&N and available at Lordco and other suppliers. It costs about as much as two of the disposable elements. The air box thermostat should be checked at least annually and replaced as necessary.

Fuel Pressure Regulator

The fuel pressure regulator is the other item that can cause trouble. The pressure can be checked with a fuel pressure gauge, costing about $25 or so (Autometer as one source from Lordco or DIY at brickboard.com. If the system is running rich, one fault might be high fuel pressure due to a faulty fuel pressure regulator. Occasionally, the FPR may temporarily fix itself simply by releasing the pressure while connecting the gauge. If the error code goes away and returns soon after the FPR is at fault. The error code of 1-2-1 will usually be accompanied by either 2-3-2 or 2-3-1. The pressure should normally be about 300 kPa; a reading of 800 kPa would be far too high resulting in incorrect operation. Examining the spark plugs may also disclose a mixture fault.

Throttle Body and TPS Service

With the help of his motorised cordless screwdriver, Ian quickly removed the throttle body from the subject car in order to inspect, clean and service the throttle body and the throttle position sensor (TPS). The removal process is quite straightforward, but with one warning: release the lock clip on plastic throttle linkage, otherwise it may break.

Once the throttle body is off, it can be inspected, cleaned and adjusted as necessary. Sometimes these get really grungy and must be scraped out but usually a commercially available aerosol throttle body cleaner and a rag does the trick. Once the entire assembly is spotlessly clean, Ian showed us that the throttle plate was binding slightly in the bore of the throttle body. The throttle plate or butterfly should not drag or bind on the side of the throttle body. It had a pronounced sticky feeling near the fully closed position. If the butterfly does bind, it can be re-centred by loosening the relatively delicate screws that hold the throttle butterfly plate to the shaft. Be careful here; if you strip one of these screws you are in trouble. Use a very good screw driver and carefully loosen the screws about one turn. With the screws loose and the butterfly closed give the face of the housing a good smack on a hard, flat surface to centre the butterfly. Tighten the screws. The results were impressive. The adjusted throttle body operated smoothly with no sign of binding.

Now that the throttle body was clean and the butterfly centred, Ian made two adjustments to the assembly. The first was to set the stop position of the throttle butterfly. The adjustment is made by simply backing the stop screw off until the screw is clear and then winding it in until it just makes contact with the stop and then add ¼ turn. On prior cars, such as those with LH2.2, this setting was ½ turn.

Next, Ian checked the TPS adjustment. This switch produces a very distinct click [play the sound, 151 kB wave file] when moving between the idle stop and the open throttle position.

Reinstall the throttle body with a new gasket and reconnect the throttle linkage. Adjust the linkage so throttle position switch to click with just a tiny bit of free play. Check that all of the ports are clear and the hoses flexible.

Once the service is complete and the engine is running again, and using a special pair of hose-clamping-off pliers, clamp off the hose to the idle air control (IAC). This should cause the hot engine to stall or, in the very least, idle below 550 revolutions per minute. If the engine continues to run, this would indicate that the idle is being fed through the throttle body and it needs adjustment.

Oxygen Sensor

The oxygen sensor is a sparkplug looking device that is screwed into the exhaust system quite close to the manifold, before the catalytic converter. The oxygen sensor can be tested by locating the sensor wire in the engine bay and measuring the DC voltage to ground. The oxygen sensor works only when it is warm. With the engine warm and at an idle, the voltage should fluctuate between 0.1 volt and 0.9 volts, centred on 0.5 roughly speaking. If it is locked at 0.1 or 0.9, that indicates that either the sensor is defective or there is a gross error in the fuel mixture.

Catalytic Converter

The catalytic converter breaks down a number of unwanted by-products produced by the engine. They operate at extremely high temperatures and are sensitive to abnormal operating conditions. They normally last ten years or so under normal driving conditions. Their failure can result in driveability issues and failure of emissions testing.

One test of the converter is to examine the inlet and outlet temperatures with the engine running and at working temperature. An infrared optical pyrometer (temperature gun or non-contact thermometer) is used to check the surface temperature both at the input and output of the converter. If the input and output temperature is the same, that indicates that the catalytic converter is not working and will often show up at AirCare as a NOx failure . The inlet temp should be 25 to 40 degrees hotter than the outlet. Once a catalytic converter starts to deteriorate, it will typically have a very short life after that. The tough part for the frugal Volvo owner is that a new catalytic converter is about $700 from Volvo. If the car is kept in tune and not allowed to run rich for long periods, it will last longer. When the turbo fails in turbocharged cars they often ruin the catalytic converter in the process. If your cat fails find out why before just replacing it or the replacement may fail as well. Aftermarket catalytic converter are cheaper but experience has shown that the may only last about a year. No one seems to know what makes the Volvo catalytic converter better and Volvo isn’t telling.

Exhaust manifold gasket leaks can usually be heard. Look for black smoky bits. Also check the flange gasket.

Ignition Components

Ignition wires should be inspected and plugs replaced. Use a set of special plug wire removing pliers to avoid damaging the wires on removal. Do not pull the leads off by the wires, always pull by the boots. The Volvo plugs come pre-gapped to the correct 0.7 mm. There was some discussion about spark plugs seizing in the aluminium heads and galling the threads on removal. This may result simply from having the plugs in the head for too long. It is wise to use anti-seize compound when replacing spark plugs. If you find a plug frozen in the hole, but very cautious of simply applying more torque to remove it.

Emergency Engine Spares
  • Fuel Pump relay
  • Alternator regulator
  • Engine speed sensor
  • Ground wire for computer.

The distributor cap and rotor should be inspected. Look at the 4 brass contacts inside the distributor cap. Scrape them clean with an X-Acto knife or good sharp screwdriver. If the contacts are badly grooved from the rotor or if the cap is cracked replace it. Lightly file the brass contact end of the rotor and look for signs of cracking on the resin on top of the rotor.

Test the Ignition leads with an multi meter and they should read around 3000 ohms. Typically, if one is bad, it will read substantially high or open.

The pump relays suffer from cracking solder. The computer module may fail to give ground to the fuel pump relay.

Recommended References

Charles O. Probst, SAE. Bosch Fuel Injection & Engine Management, Robert Bentley.

Passing Annual Emissions Tests, Paul Grimshaw.