SR20DE Tuneup and Troubleshooting Guide: Backgound Info

By Andy Radin

This is background information on all the engine systems subject to tune-up.

My experience with my own car, and what I read on the SE-R Mailing List, tells me that EGR malfunction is responsible for a large fraction of the tune-up problems. EGR (exhaust gas recirculation) simply routes some burned exhaust gases into the intake to act as a "buffer" and reduce combustion temperature under part-throttle operation. It shuts off at very low and high RPMs and at wide-open throttle to give you maximum power, so there's not much to be gained by disconnecting it. It helps keep things stable and happy the rest of the time, and keeps smog-forming nitrogen oxide pollution to very low levels.

The problem is that exhaust gases have some partially-burned crap in them - stuff that will deposit on all surfaces in the intake, and may clog narrow passages. So here's the scheme: EGR comes out of the exhaust header via a fat tube (very top of the header, driver's side) and dumps into a cast-iron piece bolted onto the intake manifold just in front of the throttle. I'll call this the EGR manifold. Several other things come out of the EGR manifold - EGR temperature sensor on some cars, the EGR transducer tube, and a galley through the intake manifold leading to the EGR valve itself. The control scheme is complicated. The EGR/canister control valve is the master control driven by the ECU, and closes at "low and high RPM" according to the factory service manual. (It's not clear exactly what speeds those are. Anyone want to find out?) If the control valve is open, vacuum is applied to the EGR-BPT (back-pressure transducer). The BPT uses the EGR pressure (from the EGR transducer tube) to modulate how much vacuum is applied to the EGR valve itself. The less exhaust manifold pressure, the more the BPT valve opens, the more the EGR valve opens. The more exhaust manifold pressure, the less the BPT valve opens, the less the EGR valve opens. When there's vacuum on the EGR valve, it opens the actual EGR galley into the intake manifold, and lets the hot nasty exhaust gases in. Whew!

There are a few trouble spots here. The passage from the EGR manifold to the EGR transducer tube is just a tiny pinhole on some engines (early ones for sure, but it's not yet clear when/if this was made larger, or if that helped). This pinhole is prone to clogging, as well as the transducer tube itself. Since a lack of EGR pressure will keep the EGR valve open, it's easy to see how this is a problem. The other big problem occurs when deposits form on the EGR valve seat, which can jam the valve open a little bit. There's a lot of rubber vacuum hoses that can crack or leak. Lastly, the EGR valve itself can become sticky and reluctant to open and close smoothly.

The idle control mechanism is the most complex of all the tune-up items, and getting the idle to be correct and stable under all conditions (cold, hot, while turning, coasting down or braking, etc) can be a hair-tearing nightmare. There are FIVE parallel subsystems for controlling the amount of airflow at idle, and it's very difficult to diagnose which of the five is responsible for any problems. The first thing to do is to do a cursory check of connectors, resistances, etc., looking for obvious problems. Failing that, the only reliable way to find the problem is to check EVERTHING in detail, one by one, skipping nothing. This will involve a good deal of disassembly and cleaning of parts off the car. Unfortunately, there is no easy way to get around this. You can always set the idle artificially high to prevent stalling around corners, but the car will just feel better when you do it right, so be prepared to get greasy and do some work. All of it is stuff that might seem confusing and hard at first, but after a day you'll be telling people "it's really easy once you know how."

The five parallel systems, in order of how much of the time they are used:

1. Air flow around the throttle plate. There is a tiny gap between the throttle plate and the walls of the throttle body. This gap tends to become clogged with nasty particles (from the EGR system piping exhaust into the intake). I don't know how much air is intended to bypass through here - some small percentage of the air handled by the others. As this clogs up, other systems (AAC) have to take up the slack, which may cause some confusion in the ECU. The ECU may be able to deal with it, but I think clean is good so clean it. Another thing to check (somewhat unlikely, but check anyway) is the throttle set screw - a little screw that sets the stop limit of the throttle plate. If this gets turned somehow, it might not let the throttle close fully, causing high idle problems.
2. Idle Adjust Screw. This is a small plastic screw in the IAA unit (pointing up) that adjusts how much air bypasses the throttle at all times. Clockwise is less air, counterclockwise is more air. This is used to set the base idle. If the base idle isn't right, the ECU has to start doing feedback control of the idle, which will be less stable and more prone to idle "divebombs" (where the idle speed drops suddenly for no reason, kicks up to high, and takes a while to settle).
3. Auxiliary Air Control (AAC) valve. This is the ECU's main idle control mechanism. It's a solenoid valve that opens or blocks airflow around the throttle plate. It's located on the Idle Air Adjusting (IAA) unit (along with the idle adjust screw and the FICD valve) on the passenger side of the engine block just behind the oil filter. The AAC valve points back toward the firewall and its wiring harness points toward the passenger-side wheelwell. The AAC valve operates a metal plunger on a metal valve seat, both of which could conceivably get dirty and fail to seal properly (or perhaps they need some sludge to seal at all. No data on this). The AAC valve itself seems robust, although it seems to get replaced quite often by dealers with undiagnosable idle/hesitation problems. I'd guess that replacing the valve is rarely necessary.
4. Idle Air Regulator (IAR). This valve lives on the bottom of the intake manifold, above the passenger-side axle. It's temperature sensitive - open when cold, closed when hot. That makes the car idle higher when it's warming up, which is good. It also has an electric heater, so it will heat up and close a couple of minutes after starting no matter what temperature the intake manifold is (which can take a while to heat up, even if the rest of the engine is warm). This seems like a likely culprit for cold-idle problems, but my little experience with this guy tells me it's very robust and not prone to collecting sludge, so it goes at the bottom of my list of stuff to check.
5. Fast Idle Control Device (FICD). This hex-sided solenoid screws into the side of the IAA unit, and opens when you turn on the air conditioning, to compensate for the added drag of the compressor. I don't know if this valve is missing if you don't have AC. Again, I think it's pretty robust so it may be safe to ignore.

Now, there are lots of other things that can have an effect on the idle speed (EGR, fuel problems, idle leaks, etc) but these are all of the direct control pathways.

Obviously, a healthy ignition system is key to a smooth and reliable engine. The important players are: spark plugs, plug wires, distributor cap and rotor, ignition coil and power transistor, battery, voltage regulator, and alternator. If those are OK, then the last thing to set is the timing.

This is covered pretty well on SE-R.net (Ignition and Spark Plugs, About Ignition Timing), the SE-R FAQ (Problems and Fixes), and some others.

My experience is that the fuel system is pretty bulletproof compared to idle and EGR, so this goes at the end of the checklist. Nissan's fuel injectors are very heavy-duty and not prone to clogging (unlike the cheap throwaway Bosch injectors). People who have sent injectors to RC Engineering for ultrasonic cleaning and balancing have been told they hardly needed it. Fuel pumps are the most likely culprit here, as there was a recall in the '93 Sentra SE-R.