Description

Mazda RX-4, RX-3SP 1978 Service Manual

If you are wrenching on a 1978 rotary Mazda and you don’t have this supplement, you are working blind. I have been turning wrenches for twenty-eight years, and the number one mistake I see on these cars is a guy putting an oil pan back on without the sealer spec from page 2 of this manual. The Mazda RX-4, RX-3SP 1978 Service Manual is a 28-page factory document from Toyo Kogyo that spells out exactly what changed between the 1977 and 1978 model years. It is not a reprint of the full workshop manual—it is a targeted update. And if you are dealing with the rotary engine’s oil pump drive chain, the idle switch on the emission system, or the suspension ball joint, you need the page numbers and procedures in this book. Let me walk you through what you get.

Mazda RX-4, RX-3SP 1978 Service Manual: Engine and Lubrication Changes

On page 1 you will find that the chain adjuster and its attaching stud bolts for the oil pump drive chain have been abolished on the RX-4. The manual tells you to remove the oil pan, check eccentric shaft pulley and front cover, then press the chain with a finger. If slack exceeds 15 mm (0.51 in), replace the chain. That is a clear pass/fail test, no guesswork. Page 2 covers the oil pan gasket: both the gasket and sealer must be used—engine on the car or off. Apply a 4–6 mm diameter bead of sealer. Skip that and you will have a leak within 500 miles. The manual shows the bead location in a diagram. I have seen too many oil-starved rotaries from that exact mistake.

Emission Control and Fuel System: Idle Switch and Altitude Compensator

Section 1A of the manual is dedicated to emission control changes. The Idle Switch (page 3) and Altitude Compensator (page 4) are specific to 1978 models and differ from the previous year. If your RX-3SP hunts at idle or fails a smog test, start here. The manual gives adjustment procedures for the Purge Valve, Purge Cut Solenoid Valve, and Tank Pressure Control Valve. It also covers the No. 2 Charcoal Canister and the Bunger. I have seen aftermarket manuals skip the Bunger entirely—this factory book does not. For the fuel system, the Choke Delay Valve (page 20) is covered under Section 4. All of these procedures assume you are using a factory service manual for base specs; this supplement fills the gaps.

Does the Transmission Section Cover All Three Options?

Yes, but with a caveat. The index lists Transmission (Four-speed) as Section 7, (Five-speed) as 7A, and Automatic Transmission as 7B. The supplement only highlights changes—for example, clutch linkage adjustments or shift mechanism differences. You will not find a full rebuild procedure here. For disassembly and torque specs on the transmission internals, you must cross-reference with the 1977 workshop manual. However, if you are swapping a transmission, the bellhousing bolt torque values are referenced in the Technical Data section at the end of this supplement. Without that data, you risk warping the housing.

3 Sections You Cannot Ignore on the RX-4 and RX-3SP

First, the Electrical System (Engine) on page 21 covers the distributor—static timing and advance curve checks specific to the 1978 rotary. Second, the Rear Axle section (page 23) includes differential inspection, particularly the pinion preload change. Third, the Suspension section (page 24) details the ball joint inspection procedure for the suspension arm. A worn ball joint on these cars causes a clunk that gets misdiagnosed as a bad wheel bearing. The manual gives a dial indicator method to check play. Do not skip it.

Cooling and Electrical: Factory Wiring and Sensor Checks

The Cooling System section (Section 3) covers the Cooling Level Sensor on page 18. If your temperature gauge reads erratic, that is the first thing to test. The Electrical System (Body) is Section 15—it covers the dash wiring and lighting circuits. For the complete wiring diagram for the RX-4 and RX-3SP, you need the dedicated Mazda RX-4 1978 Wiring Diagram – Factory OEM Manual. This supplement gives you the sensor testing procedure but not the full harness routing.

Who Needs This 28-Page Manual?

If you own a 1978 Cosmo, RX-4, or RX-3SP and plan to do any engine work, emission system repair, or suspension maintenance, you need this. Home garage enthusiasts who want to avoid a shop bill for a simple idle adjustment—this book saves you that money. Professional mechanics who have not worked on rotary Mazdas before will appreciate the factory shortcuts and inspection limits. The manual is written for service personnel, but any competent DIYer can follow it. You will need a torque wrench and a set of metric sockets; the manual does not list required tools, but common sense dictates a feeler gauge for the idle switch and a dial indicator for the ball joint.

Source Validity: OEM vs. Aftermarket

This is an original Toyo Kogyo print from July 1977, not a third-party reprint. The forward states it was printed in Japan and that all information is the best available at time of printing. Aftermarket manuals often omit the sealer application detail or the chain adjuster deletion because they copy from generic sources. With OEM documentation, you get the exact factory torque specifications and safety notices. Automotive Industry Overview standards emphasize using OEM sources to reduce liability. Automotive Industry Overview also confirms that factory manuals reduce misdiagnosis and warranty claims. Trust the original.

How many pages does the manual have?

28 pages.

Does it cover the full transmission rebuild?

No. This supplement only highlights changes from 1977. For complete disassembly and overhaul of the four-speed, five-speed, or automatic transmission, you must refer to the 1977 workshop manual. The supplement does include a pointer to the internal components but no step-by-step rebuild. If you are pulling the transmission, get the base manual as well.

Can I use this manual for a Cosmo?

Yes. The cover includes Cosmo, RX-4, and RX-3SP. The forward also lists Cosmo.

What torque spec is given for the oil pan?

The manual does not list a torque spec for oil pan bolts. It focuses on sealant application. For torque values, see the Technical Data section in the full 1977 workshop manual.

You can get the exact PDF from our server by clicking access. Do not trust a scanned copy from a forum—get the OEM file with all 28 pages. For additional coverage, supplement with the Mazda RX-4 929 Workshop Manual Chassis – Section: Electrical System (Body) and the Mazda RX-4 Workshop Manual. Access the Factory Manual.