Volvo RWD 200 Forum

The reason for this post is to provide information about the differences and potential advantages (I did say potential) of the LH 3.1 vs. LH 2.4 24x Bricks. Below is a brief list of the technical differences I'm aware of from working with both:

1: Throttle position sensing:

LH 2.4: The throttle position switch is literally two switches in one, one for the idle position, and the other for Wide Open Throttle (WOT). Beyond those two states, the ECU does not know where the throttle is positioned.

LH 3.1: The throttle position sensor has no switches and is basically a potentiometer (variable resistor), so the ECU is fed a variable voltage based on throttle position from the sensor, ie. it "knows" how far the throttle body is open.

2: Air Mass Meter (AMM)

LH 2.4: This is a "hot wire" anemometer (air flow meter) with temperature sensor. The hot wire approach is a bit fragile especially if the pre-heat thermostat is stuck open, replacement with a new unit is well over $200.

LH 3.1: This is a thin film based anemometer with a temperature sensor, it is still susceptible to the pre-heat thermostat stuck open failure, but is far more forgiving to vibration as well as oxidation caused by high humidity. New replacement cost (a year ago) was $99 for a brand new Bosch unit.

3: Cold Start Valve:

LH 2.4: It has a CSV. At high mileage especially with a plugged up spark arrestor, it is prone to sticking shut. Can be cleaned out with brake or carb cleaner, and total failure is incredibly rare (open coil on the motor).

LH 3.1: No CSV whatsoever, so no failure can be expected.

4: Fuses

LH 2.4: There is a 25A spade lug fuse under the hood which feeds the ECU and fuel pumps (aka) everything that makes the car run. Its holder (after decades) is prone to failure due to oxidation due to water ingress. Cheap to fix but a real pain along side the road.

LH 3.1: The ECU and fuel fuse(s) have been moved back inside the cabin at the base of the DS A-pillar. They have vibration failure related issues as well as oxidation, but are easier to deal with... still a lousy fuse design!

There are other minor differences, but I think the above hits the main points. Obviously though the LH 2.4 and LH 3.1 have different ECU units and harnesses for that matter. There are "general assumed" differences between the two systems:

LH 2.4: Doesn't do too well in the MPG category, perhaps 26 to 28 MPG v.s 28 to 30+ MPG on LH 3.1 (additional confirmation requested). LH 2.4 parts are available new and used almost everywhere at low cost (except the AMM).Parts are easy to find... this is a GREAT thing!

LH 3.1: Does a bit better on gas mileage due to "predictive" fuel management vs. LH 2.4. On the serious down side, LH 3.1 ECUs bring top dollar, working or not! Very few of them were used in the US, starting in 91 or 92 and only on manual trans cars (except for one auto trans that I know of). So the LH 3.1 system is very rare... walk to the dealer and bend over!

Please note, the above post was not intended as a which system is better, as in my mind they balance out fairly well. That being said, I encourage others who know both the LH 2.4 and LH 3.1 systems to add to this post and our understanding/knowledge! Please also note, both systems use the same failure prone fuel pump relay and fuel pressure regulator!

Hope the above is found to be helpful!

jorrell
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92 245 250K miles, IPD'd to the hilt, 06 XC70, 00 Eclipse custom Turbo setup...currently taking names and kicking reputations!