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SR20DE Tuneup and Troubleshooting Guide: Tune-up Checklist

By Andy Radin


I’m going to start with the easy tune-up items that are regular maintenance jobs that ought to be done every so often no matter what. Then I’ll move into specialty checks that only need to be done in search of a specific problem, since they’re more difficult.

I will try to include specific check data on every component in the chain, but that may take some time to complete. Most of it will be out of the factory service manual for the 1993 G20. Note that there has been a lot of variation in emissions-related controls on the SR20DE over the years, so some of this information may not be exact for your car. In that case, I’m afraid you will have to use your brain and other resources. Please submit data that extends or contradicts anything written here.


These are straight regular-maintenence tune-up items. Check the owner's manual for service frequencies. Most of us take care of this stuff a little more often, just to be sure and because we kinda like it.

  • Check/replace spark plugs. For info on reading spark plug colors, click here. If this indicates lean running, suspect fuel problems. If this indicates rich running, suspect vacuum leaks, O2 sensor, and ignition. If the plugs look just right, suspect EGR and timing. Note also the debate on spark plug brands - Bosch vs. NGK. DO NOT test spark by disconnecting spark plugs wires while the engine is running - you can easily ruin the cat by flooding it with unburned gas.
  • Check/replace spark plug wires. The standard method is to use a spray bottle of water – spray a fine mist over the wires while running the engine. If you see any sparks from the wires to the valve cover, replace. Check that the rubber insulator is still flexible and soft. Check the resistance with a multimeter, if any are unusually different, replace. Read Problems: Low gas mileage for an idea of what to look for.
  • Replace fuel filter (every 30,000 miles). Mine once got nuked by a single bad tank of gas – moral: don’t fill up at a station while the tanker truck is refilling the tanks. All kinds of nasties get stirred up and pumped into your car. Follow the instructions on Changing the Fuel Filter. Be VERY careful doing this, as lots of gas will spill. Be fire safe – unplug the battery, have an (automotive rated) extinguisher nearby, have adequate ventilation, watch your eyes.
  • Clean or replace air filter.
  • Replace distributor cap/rotor. These are strong, definitely good for 60,000 miles apiece. Remember the Wayne Cox HP trick.
  • Test battery voltage. For me, getting a new battery (old one was 6+ years old) gave me more power, smoother idle, smoother running, and fewer cranks to start in the morning. I got an Optima; your results may vary. Any auto parts store has a heavy-duty meter that can tell you voltage and current output, check that first! Please recycle the old battery if you replace it.
  • Test alternator voltage with a meter across its terminals. This is likely to either work perfectly or not at all, so it’s unlikely to affect the engine if there are no other electrical problems.
  • Replace O2 sensor, probably every 60,000 miles. O2 sensor function can be checked via the ECU’s self-diagnostic mode, which is a neat test. The ECU is in the center console, below the stereo/HVAC and in front of the shifter. Remove the front-most inside kickpanel on the passenger side. Behind that is a sturdy, rectangular metal box. That's the ECU. In my G20, on the passenger side, there's a cutout covered by a little sheet of plastic. Just behind the cutout, toward the back of the car, is a screw that sets the diagnostic mode. Turn the ignition to ON (do not start the engine). (This screw seems to be located elsewhere in other cars - activate brain at this point to find it.)

Turn the small screw fully CLOCKWISE for at least two seconds, then turn it fully COUNTERCLOCKWISE. (You have to redo this every time you turn off the power to get the diagnostic mode. Make sure not to drive around with the screw set CLOCKWISE, always set it back counterclockwise!) Start the engine, and warm up the O2 sensor by idling at 2000 RPM for about 2 minutes. The CHECK ENGINE light will now flash with the O2 sensor condition: ON = lean, OFF = rich in closed loop, if it stays one or the other it’s in open loop.

At 2000 RPM idle, Nissan claims it should switch on and off more that 5 times every 10 seconds. I think that is WAY conservative – it ought to be switching a LOT more than that. Drive around with the diagnostic on to get a feel for when it’s in open and closed loop, and to make sure it gets really hot. I think if it only goes 5 times in 10 seconds, your sensor is HOSED and probably has been for a while. Since this sensor is kind of the kingpin of the whole mixture control system, I tend be on the liberal side with replacement. There seems to be sentiment that anti-seize compound can ruin O2 sensors, so keep that in mind.

Another note on O2 sensors: the sensor is designed to be grounded through the exhaust header – there’s a ground strap to the body just before the catalytic converter. If you have an aftermarket ceramic-coated header, there may be a poor electrical connection through it. Check the voltage on the sensor body at hot idle (careful of burns). So far, there is no data on what is acceptable. You may wish to add a ground loop regardless. I used a big cable clip from a surplus store – if you can find an enormous ring terminal to fit around the sensor that would be great too. Run it to the ECU ground on the intake manifold, behind the throttle body. Use gold-plated connectors if you’re hardcore (audio guys have them lying around, ask your friends). I don’t know for sure what effect this may have, still collecting data.

  • Check MAF ground. If you have a ’93 or earlier SR20DE and this has never been done, it’s a prime suspect. Plenty of help here: MAF Sensor Grounding.
  • Clean the throttle body. Careful! Don’t just fill the throttle with cleaner and think you’re done. Read Cleaning the Throttle Body and Part Three under Problems: Erratic Idle and Jumping Idle. If you just soak the throttle/manifold with cleaner and burn it off, you WILL coat your pistons with gross stuff.
  • Set idle speed. Follow the instructions on Idle Speed Adjustment. Here’s a dilemma – some people have reported needing to unplug the AAC valve to be able to set the idle at all (read about that under Problems: Erratic Idle and Jumping Idle). However, the FSM specifies that the idle speed should drop when unplugging the AAC valve so it’s hard to imagine that you can set it right while unplugged. If it will set to the right speed, but still has divebomb, stall, hunting, etc. problems, then something else is wrong for sure. If you don’t have time, or give up, then try the following. Set the idle according to standard procedure. Drive around for a while – if the problems persist, open the idle screw a quarter turn (no need for the full TPS disconnected, rev three times deal). Drive around. Repeat. When the engine idles too high at stoplights, or starts mysteriously speeding up at idle, close the screw a quarter turn. Repeat as necessary. This will take a while, so keep a screwdriver in the car. You ought to be able to find a compromise that won’t bother you too much.
  • Set base timing. There are instructions at About Ignition Timing. Another tricky one – the timing needs to be set at exactly the base idle. Since the timing advances more and more with higher RPMs, it’s only calibrated to 15-17 degrees BTDC (your preference) very near the 750-800 RPM base idle. Setting the timing to 15-17 when the idle is too high is like retarding the timing, and vice-versa. The manual tells you to set the timing before setting the idle – I think that’s crap, for the above reason. Set the timing after the idle.

The following tend not to be regular tune-up items, but if there’s a persistent problem and you’ve done all the above or are SURE that none of the above have a problem (and be careful – this can bite you in the ass because you’re almost never SURE), then continue.

  • Clean EGR-BPT tube. I love this one and the next, because it’s easy to do, practically free, and is the cause of a LOT of frustrating problems. Start at Problems: Surge and hesitation. (Also, Problems: Check Engine Light may apply to you.) My only beef with this procedure is that some cars have only a pinhole opening between the EGR transducer tube and the EGR manifold. This procedure doesn’t actually clean out the pinhole. If the wire doesn’t push all the way through, try this: obtain about a foot of vacuum hose. With the EGR-BPT valve removed, attach your foot-long hose to the EGR pipe. Make sure the engine is off, and cold. Spray several squirts of throttle body cleaner into the hose. Let soak a few moments, then place your lips over hose and blow the cleaner through the pinhole. Let the excess evaporate for several minutes if you plan to start the car.
  • Clean EGR valve. Same procedure at Problems: Surge and hesitation, at the very bottom. When you take off the EGR valve, you will instantly see the problem (if your car’s like mine, that is). Below it, in the EGR passage in the intake manifold, are all kinds of sticky black particles all stuck together. It’s not hard to imagine that some of these nasties blow through the EGR valve and get stuck, jamming it open. Be careful stirring those guys up – it’s hard to get rid of them all, and loose ones will just make it worse when you start up.
  • More things to check in the EGR system if the above two don't fix anything: there should be vacuum on the right hose into the EGR-BPT, and vacuum through the BPT into the EGR valve. Check both (engine running, medium revs). Continue working backward - check vacuum both sides of the EGR/canister solenoid, check that the solenoid is operating, check for power on the solenoid, check for vacuum leaks in all the little tubes.
  • Check PCV valve. It ought to rattle freely, with no sticking. Cheap to replace. Use a little high-temp teflon plumber’s putty to seal the threads.
  • Check for intake air leaks. There are four tubes leading from the pipe between the MAF and throttle (three if you have a replacement intake or no PAIR smog pump stuff on the front of the engine). One leads to the valve cover, one to the PAIR (if you have one), and one to a tiny metal tube. The last one (it’s pretty big, maybe inch) supplies air to almost all the idle control devices – IAR, AAC, FICD, and idle screw. If you pinch this closed with pliers, the only air getting to the engine is that going around the throttle plate gap. The engine should DEFINITELY stall if you do this. If it doesn’t you have BIG problems with your throttle or intake manifold. The FSM has a few diagnostics for intake leaks: Disconnect the O2 sensor while running the engine. Hold the engine at 2000 RPM for 30 seconds. If the speed drops when you release the throttle, there’s a leak (remember to reconnect the sensor). Or, pinch the PVC hose over the injectors closed with pliers. If the engine speed rises, there’s an air leak. If there’s a leak, tighten all hose clamps, look for dried-out, cracked hoses. It’s not a bad idea to replace all the vacuum hoses anyway…
  • Adjust the throttle position sensor (TPS). Instructions at TPS Voltage Adjustment. I think this is pretty key for the surge/jerking at very low RPMs. The ECU has a TPS voltage threshold at which it decides the throttle is fully closed. If you’re at speed and this happens, the ECU cuts off fuel totally while you coast down. When the speed drops enough, it decides it’s time to go into open-loop idle. If the TPS voltage is off, you might hit this threshold when you’re rolling in traffic. The engine will start surging at it clocks across the threshold – very annoying and it makes you look like you can’t drive a stick (if you have one). If it’s set right and still surges, keep going.
  • Check that the PAIR is not leaking into the intake. Read the portion about the PAIR valve at Problems: Erratic Idle and Jumping Idle. This won’t apply if you have no PAIR valve (later cars), or have an intake with the baby K&N.
  • If you’ve gotten this far and still have cold-idle hunting/hesitation problems, it’s time to check out your idle air regulator. The way to test it is a pain. Jack up the whole front of the car when it’s stone cold, so that there’s plenty of clearance around the front wheels (use jack stands, chock the rear wheels, set the parking brake, clear everything away from the front wheels. Do not just use the jack!). Check your jack stands again! Get a light, crawl under and look on the underside of the intake manifold, right above the passenger side axle. There’s two hoses with a valve connecting them – that’s the IAR. Crawl out, start up the car and leave it running in neutral. Work quickly because you need to be under the car while it’s still cold, and you only have about a minute. Be absolutely sure it’s in neutral, and won’t get knocked into drive. Check the jack stands again - if it’s still firm, crawl under the car. Look up at the IAR, and pinch the left hose closed firmly with pliers. The engine speed should drop, because it’s supposed to be open when cold and you just shut off its air supply. Now warm up the car for five minutes at 2000 RPM (neutral!) and pinch the hose closed again. If the engine speed drops, it wasn’t closed. Check that there’s power on the harness for five seconds after the ignition is switched to "ON" (requires a friend), and ground on the other pin. If it’s the harness, you’ve found the problem. If not, and it failed either pinch test, you need to remove the IAR and test/fix or replace.

For instructions, click here. My experience, when I did this, was that the damn thing was as clean as the Waldorf-Astoria inside, even with 120,000 miles on it. I think it’s tough. I have yet to hear about one that was actually blown or clogged.

  • Well, we’re getting towards the end here. It’s time to go after the big guns – the AAC valve, FICD, and idle air screw. All of these together are attached to an aluminum "box" that bolts to the side of the intake manifold – the Idle Air Adjusting (IAA) unit. It’s above the belts, behind the oil filter (toward the firewall). There are four 10-mm bolts holding it on. Two of them are really, really hard to get to (on the G20, may be easier on other cars with smaller shock towers). I had to move all the wiring harnesses out of the way, as well as the alarm horn (G20 specific). I had an easier time removing the FICD (hex-shaped brass cylinder sticking sideways out) first to allow better access to the two bolts on the bottom.

WARNING WARNING WARNING: I found out the hard way that there are spring-loaded parts inside the FICD that will pop out when you do this. If you're ready, you can catch them. If you lose them, you will NOT find them so be ready.

Once you remove the IAA body, you can remove the AAC itself with a Phillips screwdriver. Both the AAC and FICD are solenoid valves - there's a spring-loaded steel plunger inside that opens or closes against the valve seat. The plunger, spring, seat, and housing will all be dirty. Clean them with throttle body cleaner, WD-40, or other solvents, and Q-tips.

The AAC should have a static resistance of about 10 ohms between the terminals. The FICD will be about 25 ohms. You can also check that they work by applying 12 volts across the terminals WITH THE PLUNGER AND SPRING IN PLACE – do it only briefly and not too often (don’t know how much current they’re rated for, and you don’t want to burn them out). The plunger should click inward when you do this.

You will probably need a new paper gasket (AAC gasket? Or IAA gasket? Work with the parts guy/gal here) to put everything back together - the one on mine disintegrated. It was only $1.10, but be sure to get it first as it's not a common item. Remember to plug all the harnesses back in, and reset the computer for an hour before running because its adaptive idle control will be SCREWED with a clean unit.

Remember the big warning - the spring loaded parts inside the valves will try to leave when you take them apart. If you lose them, you have to buy a new unit ($50 for the AAC, $150 for the FICD).


There are some things to check in the fuel system. However, any real problems would be much more serious than simple idle or tune-up problems. So let's look at minor problems:

  • Check that there's vacuum on the fuel pressure regulator, on the end of the fuel rail.
  • You can do a DIY check that all your injectors are running. With the engine running at idle, disconnect injector harnesses one at a time. The engine speed should drop the same amount for each one. DO NOT test spark by disconnecting spark plugs - you can easily ruin the cat by flooding it with unburned gas.
  • You can do a quickie test on the fuel pump - you should feel pulsations in the fuel line for about five seconds after turning the key to "ON," as the fuel pump primes and pressurizes the fuel system. Quick note here - someone once told me that his car wouldn't hunt on cold start if he left the key "ON" for ten seconds before starting, to let the fuel lines come up to pressure. Worth trying...