Seafoam is sort of a gimmick.

The CRC brand AMM / MAF cleaner is sort of suspect as gimmick.

As Art Benstein points out repeatedly in many posts, when you shut the engine off, the ECU sends current to the AMM hot wire to burn off material off the platinum alloy wire.

Your (all) 1988 (factory) Volvo 240 (does not include Turbo).

You are in California, yes? Is your 240 a Cally-market 240? Your 240 will then have EGR. Though the EGR system *probably* works. The system requires service as exhaust gas passed through the system into the air intake through the EGR valve condensates and clogs EGR from the air intake port backward. Got one of these (from my 1991 Volvo 240 Germany market 240 required EGR then):



You have factory EGR. I'd address tune first.

Though age, faulty air filter, the engine barfs up petroleum products that ooze down the accordion-style intake hose between the throttle body and the AMM / MAF outlet backside (flame trap hose routing, failing oil ring or compression ring allowing a lot of oil sump gasses to pass down the intake), bio guck (air filter), or rodentia may mean some time with checks on the air induction intake from (air filter box inlets - you have two - cold and preheat - the preheat air box flap valve stuck to hot all the time not mentioned in responses to you). The AMM / MAF can get loaded with oily guck, so some careful CRC AMM / MAF cleaner may help.

I use the BrickPix. Say not to PhotoBucket.


Compression or leak down test to verify well sealed piston rings and not excess blow by.

High NOX suggests a lean running condition.

There are several potential causes for high NOx emissions from a vehicle's exhaust. A search on evil Google for "High NOx causes" reveals a list:

Air-Fuel Ratio Problems. High NOx emissions can occur when an engine's air-fuel mixture is too lean. ...
Engine Timing Problems. ...
Catalytic Converter Problems. ...
Improper Exhaust Gas Recirculation. ...
Carbon Buildup. ...
Overheating.

https://itstillruns.com/causes-high-nox-emissions-7619425.html

https://www.smogtips.com/failed-high-NO-nitric-oxide.cfm

Air induction intake vacuum leaks after the air filter and more so after the AMM / MAF as the accordion hose may have holes (remove, inspect, degrease, seal with adhesive), leaking air intake port manifold gasket (these get brittle and shrink with heat and age), or another other point that can fail with age and underhood conditions (heating and cooling and oil and vibration / chafing). The PCV system can form leaks at any union, the breather box can crank through the o-ring under the beather box that seals the ABS breather box to the engine block can become brittle, shrink, or other wise have cause to lose seal. I remove the large PCV hose from the union near the throttle body, remove the small PCV and clog it, and with oil fill cap secured on, blow. Should not or barely be able to move any air with good lung power. With the oil fill cap off, lung power can easily displace air in the sump through the oil fill port with oil fill cap removed.

Other cause of lean running condition is the preheat flappy valve in the air filter box upstream of the filter. A bi-metallic spring controls this flap valve and on 30-year old Volvo 240, non Turbo 700-900, the all fail to hot air all the time. Can ruin the AMM / MAF through a hot summer or two.

If not in a state that inspects emissions, remove the silver accordion hose between the exhaust port manifold and the air filter box lower (preheat) air inlet. You can remove the assembly if in a temperate climate. Else, merely replace the thermostat. After market versions from iPd or FCP Euro replace every two years or so.

Wahler (Borg Warner) and Calorstat make these. Dunno what is OEM.

Calorstat at iPd
https://www.ipdusa.com/products/5451/103083-airbox-thermostat-valve

Wahler (Borg Warner) at FCP Euro:
https://www.fcpeuro.com/products/volvo-engine-air-box-thermostat-1266826

Dunno what makes OEM. Yet here is the Tasca entry on it:
https://www.tascaparts.com/oem-parts/volvo-thermostat-1266826

If you mean to replace this, takle care removing the assembly as the assembly clips to the air box housing in three places.

When addressing engine tune issues, I start at the intake and work downstream to the exhaust unless there is an exhaust leak. Usually.

As 1988 does not offer OBD, it may help to test sensors like the ECT, AMM, knock sensor, and so forth for continuity and resistance. Bentley is better than Haynes. Factory green manual is best for the practiced mechanically inclined.

Questions?

Hope that helps.
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