My 95 Corolla (1.8L, automatic, 150k) failed the NOx reading in the emission test. The HC, CO, CO2 readings were very good. I replaced the converter, back-flushed the engine, replaced the thermostat but it still failed. I took it to a mechanic but he was not able to find out what was wrong so I took it back after two days. I did a lot of bench tests and road tests (by introducing vacuum and voltage signals into the cabin), and found the EGR valve, the EGR vacuum modulator (regulator), the EGR solenoid vacuum switch, the Oxygen sensor, the temperature sensor for the computer and the temperature switch for the fan were all good. In the end, I found out there was no exhaust gas going into the EGR. I made an air tight connection with a tube into the exhaust outlet hole for the EGR at the intake manifold, revved up the engine and blew into it. There was little resistance other than that caused by the restriction of the small diameter of the tube itself. It seems there is a leak somewhere in the exhaust path before the EGR. I want to take down the intake manifold to take a look, but want to have some basic idea about what I might be dealing with before I start. Can someone tell me what may possibly be the problem? Is it likely located in the intake manifold, or likely to go beyond, at the engine head or the exhaust manifold? And what might it be--gasket or metallic part failure? Should I be prepared to buy another intake manifold or even the engine head? It doesn’t look like the exhaust was from the exhaust manifold but I simply have no idea where the exhaust came from. I have done many small car repairs but have not done anything this big.
A little more info about the exhaust leak: When I applied vacuum on the EGR, at 2 inch vacuum the engine rpm started to rise in proportion to the vacuum I applied. It continued to rise until about 8 inch then it wouldn’t rise anymore. Instead, the rpm went into a cycle of going up then down then up then down. It feels like the computer was trying to correct something that it detected but this resulted in a loop. By the way, the check engine light never came on. The rising rpm makes me think maybe the exhaust path was clogged somewhere and zero exhaust went into the intake, but on the other hand there might be a source that supplied air into the intake when the EGR was opened.
Also with this big job I am still not sure I will pass the test. A Toyota mechanic said that at 15 or 25 mph the EGR need not come on. Could someone confirm that the EGR should be on at 15 mph, cruising?
On your year the EGR is controlled by the ECM....if it doesn't open suspect a faulty ECM!
September, 12, 2007 AT 3:36 AM
Are you ready for information overload???
I think by this picture the flex tube is comming out of the modulator, but you can tell better than me.
Hope this info helps!
September, 16, 2007 AT 3:38 PM
Hi merlin2021--thanks for replying.
The tube that looks coming out of the modulator does not supply exhaust to the EGR. To the contrary, exhaust gas goes out (it doesn t actually flow) from the EGR to the modulator through this tube.
There is no flex tube to supply exhaust. The EGR is bolted onto the intake manifold. Its bottom has two airway openings. Exhaust gas enters the lower opening into a pressure chamber inside the EGR base, then passes around the EGR valve when it opens, and goes out of the upper opening into the intake manifold. Unlike Ford cars that use a flex tube to get exhaust from the exhaust manifold (as you said), there is no flex tube here. Exhaust comes from a path that is built-in inside the intake manifold, but I have no idea where it comes beyond that.
The pressure chamber inside the EGR valve base has an opening that lets exhaust gas, through this tube, to pressure on a diaphragm inside the modulator. The modulator s P port gets vacuum when the throttle is opened but not too wide. Inside the modulator, P is connected to the atmosphere and vacuum is destroyed and can t reach Q (the output port) if this connection is not shut off. With sufficient exhaust pressure, the diaphragm will shut off the atmosphere connection to P so that vacuum will reach the EGR valve to open it. Port R gets vacuum when the throttle opens wider. R is not connected to the atmosphere and vacuum always passes to Q.
Toyota did not explain this in its materials, so I think it s worth sharing.
Now back to my own question. Right before I was to replace the modulator (as suggested by Toyota s materials), I found out there was no exhaust pressure at the tube, then found out exhaust never reached the EGR. There is a leak for sure, but it might be a combination of clog and leak. However, I have not done any big repairs and have no idea about the structure, components and weakness of this EGR exhaust path. I hope to have some guidance before I start (or decide to give up). Ideally, the question is best answered by someone who have worked on this model and have paid attention to the EGR exhaust source path that possibly involve the intake manifold, the engine head and the exhaust manifold. Any suggestion about what might be wrong inside these three parts will be helpful. You may or may not be very familiar with such a specific area, but I wonder whether it s OK with you to let or to ask other moderators to jump in?