Honda Prelude Forum - PreludeDriver.com

I'm reviving a 89 B20A with PGM-FI and now have it up and running. Fixed the head and replaced the timing belt after the previous owner did something wrong. Cleared all the engine codes and now have no warning lights, and have the ignition timing dead on. Took it to a smog shop today and found that the CO (carbon monoxide) levels were very high at 15 mph (.55 is max, mine was 9.5!). At 25 mpg it did better, but still high (.46 is max, mine was 1.25). The previous owner put on a California legal DC sport header, which only has one 02 sensor. He joined the two wires from the previous two sensors together - it seems to work, at least it does not throw a code. Could this, however, be the reason I am getting such high CO readings at 15 mph? Normally why are there two O2 sensors? Should I just replace the sensor and hope for the best (replacing the 02 sensor seems to be the main advice to start to fix high CO levels), or are there other obvious reasons why CO readings would be so high? Thanks for the help.
Ken

There are two oxygen sensors to test the oxygen levels at multiple positions of the exhaust tract.

I'm going to guess that your troubles might be related to the way the oxygen sensors have been wired up together. I don't see how that would be conducive to accurate readings. Not knowing your exact setup, it's hard to suggest a solution, but I would do whatever you can to get the O2 sensor setup as close to stock as possible. Obviously hold on to your DC sports header, as that's rare find, but work from there.

thanks much for the quick reply. As far as I can tell all that they did was to wire the two singles wires (are they single wire? - I didn't look close) together with a wire nut and then take the single wire to one 02 sensor, which is now located downstream on the exhaust, under the car, not on the manifold itself. Since it is a Calif. legal header I assume that this can work, and I know it passed smog with it on about 4 years ago, the last time it was checked (it sat for over two years). I thought I'd start by just getting a new 02 sensor and trying it; if that doesn't work, I'd go to pic-a-part and pull out a regular exhaust manifold, hopefully with the sensors still on them, and go from there.

I don't know much about DC headers - they are good?

DC Sports headers are probably one of the most sought after parts for the 3rd generation Prelude. Hold onto yours like it's gold. You're very lucky to have one.

Since the car has been sitting for such a long time, a new O2 sensor might be a good idea.

Very interesting about the DC Sport Headers. I bought the Prelude from a guy who had given up putting it together after he couldn't get it running following his attempt to redo the head. It has a bunch of mods - I'm not really into them too much but it is nice to know that this one is valuable. It also has to Tokiko struts and (I believe) lowered/taut springs, H-rated Kumho tires on some very nice alloy rims. It needs to have the original black paint redone (just grey primer at the moment), and generally needs some love and care, but it appears in good shape. I had the head redone and then put on new timing belt, water pump, and assorted other little new pieces on the engine so that my son (who will drive the car) wouldn't have to worry too much about replacing stuff. Now if I can just get it to pass smog!

Well, the saga continues. I'm still trying to pass smog in California and nothing has helped too much. The problem to be fixed is the CO readings.
They are way over the top, indicating too much fuel. The emissions are fine at idle but under load (15 and 25 mph) the CO readings go far too high. So far I have:
-put on new water temp sensor
-bought two new oxygen sensors
-replaced the DC Sports header with a stock exhaust manifold
-replaced the ECU with another (from a junk vehicle)
-replaced the MAP sensor (with another from a junk vehicle)

All of these things have made a difference but not enough of one to pass smog. As mentioned, the car passed smog about 4 years ago (with the DC header on). Previous owner had put on a new catalytic converter to pass smog last time. But then the car sat as the previous owner had a cam rocker go wild in the head after replacing the vehicle speed sensor (LONG story!). I rebuilt the head, put everything together, fixed some minor stuff. Now no engine code lights, runs and idles great, but I can't get it registered until I get the CO levels down. One thought it that something is wrong in the fuel injection system, perhaps leaking injector or problem with fuel pressure. But the fact that CO level was o.k. at idle seems to indicate the problem is more complicated. Thoughts?!!

Are the injectors the stock size?

thanks for the reply. Good question - no idea, but I wouldn't be surprised if they were not given how many other mods this car had when I bought it. how does one check? Even if they were larger than normal, wouldn't the MAP sensor try to always keep the correct air/fuel ratio at 14.7/1? Or am I thinking wrong about how the ECU looks at larger injectors? I could easily pick up some used stock ones at pick-a-part. I was next going to go after the fuel injection system, but wasn't sure what exactly to go after. The mechanic I am consulting felt that if the CO numbers were o.k. at idle (which they are) it would indicate that there was not a leaky injector, but I was still wanting to check it out. On my old 75 Saab 99LE with mechanical Bosch-Jetronic FI one could simply take out the injectors and hold them over a bottle to see if they leak - I would assume things would be different on this system.... How does one check for leaks, or does one just follow the diagnostic tests on page 6-190 for FI electrical problems? I'd *really* like to solve this, as the car deserves to be back on the road after all that I've done to get it going again!

The ECU delivers a pulsewidth to the injector to open it. If the injectors are over-sized the MAP sensor and O2 sensor will try to scale back the fuel but scaling back an oversized injector enough might be beyond the ranges of the programmed fuel map.

To figure out what type of injectors you have, you'll just have to see if they have anything written on them. They should at least have a flow rate marked on them somewhere.

I think it would be pretty hard to test the injectors for leaks. You could hold them over a jar, but you would have to hold all 4 of them into the fuel rail (with the fuel rail off the head) to be able to do this. That's going to be very difficult with the system pressurized. Having a proper flow bench is really the only correct way to test an injector.

If you really wanted to check for leaks, it might be easiest to pull the plenum of the intake manifold off and see if you can fashion a probe to run down the intake runners while the system is pressurized. Attach a Q-tip to the end or something and see if you get any wetness.

thanks much - re the oversize injectors, that is exactly what I was thinking - the O2 and MAP sensors can only deal with so much variation. I think that is the next thing for me to pull and check one of mine - perhaps go to pick-a-part and pull an injector off a car there to compare it with.

I now really doubt there is a leaky injector, as the CO level is apparently o.k. at idle - it is only a problem under load...

After a long search, I finally found the problem and the car passed with no problem. The fuel injectors were not it, as I replaced them and the emissions were still lousy. The problem was that the engine was never going to 'closed loop', where the 02 sensors were being used to obtain the maximum air/fuel efficiency. This was because there was a short in the both of the wires. The previous owner had installed a DC header and just spliced in wires together to make the 02 sensor 'work' with the header (which only uses one 02 sensor, and relocates it under the car). On the 02 sensor wires there is a braided ground wire that surrounds the main wire. If you don't know that when you try to repair the wiring you can easily wire both the ground and the main wire together. He must have used a disreputable shop to pass smog, as there was no way that it could have. It took me a bit of work to discover it but it is all good to go now. The fuel injectors were not a problem at all - it passed with .05 CO content at 15 mph and .00 CO at 25 mpg. Not bad. Thanks for all the help!

Really glad to hear you're back on the road.

Thanks for following up on your progress for the benefit of others.

Do you plan to put the DC sports header back on your car, it would be a waste if you didnt use it. I would keep it and only sell it if you were absolutely not going to use it. Its getting harder and harder to find those rare parts. Glad you get to enjoy your car!

Funny you should mention that. I'm fixing up the car for my son and he is basically broke so he agreed to sell the header since it is not on the car. I had the shop cut off the pipe with enough room on the far side of the flex pipe to weld the next piece on. I just put it up in the 'for sale' section : CARB legal DC Header for $200 +$25 shipping. That seems like a fair price given what I saw for other DC headers on Ebay but if someone thinks otherwise let me know! Here's a picture of it