Hello everybody. I'm fighting the dreded P1491 trouble code problem. Here's where I'm at:
1997 Honda Accord 4cyl, AT. (neighbors car). He has a P1491 trouble code. The CK engine light had been on for quite a while but he was not concerned with it until it was time to get the car through emissions (which is wont do). Monts later he broke the timing belt while on the interstate. The car had 240K miles on it and we decided to just replace the engine rather than remove the head and replace valves and such. I located one with 68K miles and installed it for him. The check engine light came on shortly after. I checked the code and it was P1491.
Here's what I've done to troubleshoot the code so far.

Removed the EGR passage cover plate on the intake. All the holes were nice and clean (hence the 68k mile engine) but I cleaned them anyway.

Removed the EGR valve, cleaned the port there as well.

Tested the vacuum diaphram in the EGR valve to make sure it actuates with vacuum (it does). I also swapped it with another EGR valve from matching Accord with no issues.

It was only after this last test did it hit me.............the same code was present with the old engine (and I did clear it with my MT2500G scanner). When I swapped engines I swapped them with intake AND EGR valve. Now it has me thinking the problem is in either the vacuum system or the CPU.
I've already searched for obvious vacuum leaks with no luck. What I need now is a vacuum diagram so I can verify they are routed correctly. I would also like some specs on how much vacuum should be reaching the EGR with the RPM's up. I'm guessing that I might have a bad EGR vacuum solinoid or something. Where/when does the EGR solinoid (located on drivers inner fender) get it signal?
I'm pretty handy so I'm up for any factual info you can share. I have no problem with test sequences and have most any tool needed to perform them.

Looking forward to some insight from somebody who's been down this road before.

Once again;

It's not the EGR passages
It's not the EGR valve

I need a vacuum diagram
I need information on the EGR solinoid

Thanks

I searched this thread and found other people dealing with this problem but no real resolution.

 

The 94 shop manual can be downloaded from the online manuals thread in the DIY section. The vacuum routing should be the same. The ECUs are different, so the egr testing diagram for a code 12 is the same except that pins will be different on the PCM.

The first step: If you have a vacuum gauge, hook it up to the vacuum hose going to the EGR valve. There should be no vacuum on that hose when the engine is idling.

 

Heres the hoses to the EGR valve.

Also there should be a vacuum hose diagram pasted under the hood somewhere, probably part of the EPA certification sticker.

 

Ok, here's what I did and what I found. I tested the EGR by hand to make sure the engine stumbled and it did. As mentioned in previous post I cleaned the EGR ports out already. I checked to make sure the vacuum lines were hooked up correctly and they were.
I then started tracking vacuum from the intake through the pod then into the EGR solenoid. No vacuum was coming out of the EGR solenoid even with the engine RPM's over 3000. I removed the solenoid and cleaned the ports as good as I can. Just for the heck of it I wanted to make sure my vacuum lines were good from the the solenoid to the EGR valve. I tried to suck open the valve by mouth via the line that hooked to the solenoid and nothing happened. I then realized I had a blockage inline somewhere. Using the air hose I blew backwards from the EGR through the lines and "BOOM" a ton of dirt came out. Aparently, with this being a junk yard purchased engine a bug of some sort built a home inside the metal line between the EGR and EGR solenoid. I then unplugged the EGR solenoid and used my Fluke meter to test for voltage to it. As I increased the engine RPM the voltage increased so I know I was getting signal to the EGR solenoid. Based on that I decided the EGR solinoid was bad. I found one at a local junk yard (got two actually) and put it on the car. He has driven the car about 80 miles so far and the light hasn't come back on. I'm guessing I fixed it, or at least I hope so.
So, in short form, I had a bad EGR solenoid and a blockage in the line between it and the EGR valve.
Now I have another problem. The engine RPM's jump up and down as if it had a bad IAC valve. Unlike the Fords where you simply unbolt and replace it, I think this one works off water temp where it passes through the throttle body? I'm guess guessing on that so if anybody has insight they want to share let me know.

by the way.......Merry Christmas to everybody!

 

Back from Christmas break...

Unplug the wire from the IAC valve - the idle should drop down to about 550 or almost stalling. OTOH, the fast-idle thermo valve (FITV) underneath the throttlebody has coolant hoses. Search for FITV & look for instructions about removing the cover & tightening something inside.

 

I didnt think that model had an electronic IAC valve. I will check it out. He called me yesterday to tell me the check engine light is back on. I havent scanned it yet to see what it is though. I'm guessing EGR flow issues again. There's a chance that the salvage yard soleniod I put on is bad. I never tested it, just put in on and sent him on his way.

 

FITV is #14 under the throttle body.
IACV is #19 on back of intake manifold.

 

As instructed, I unplugged it and the RPM's dropped down but it was still running. As soon as I plug it back in the starts the RPM jumping again. I removed the FITV and cleaned it as well as tightened the slotted end as instructed. I also tested the internal thermostat with heat and watched it move. I blew out all the coolant lines, both to the IAC and to the FITV. This is a very low mile engine so everything was pretty clean but it was perfectly clean when I put it back together. All back together and the same problem. I adjusted the idle screw up and when my idle hit about 1700ish the RPM jumping quit. As soon as I tried to idle it back down it started again.
Do I have a bad IAC? What else could it be??? Everything is clean, tight and and tested.
For now I have it idled up to 1700 for him to drive. Hopefully it wont pop an IAC code this time and he can get it through emissions.

Looking for more feedback here. I'm thinking I need a new IAC but dont want to take a $189 shot at it. I could rob a few off cars in junk yards I guess.

HELP!

 

Strange idle problems can arise from air in the cooling system. Did you bleed the air from the system when you refilled the coolant?

 

Yes, I did bleed the air from the cooling system after the thermostat opened. I even checked the hoses in and out of both locations to verify they were all hot.
I will bleed the system again today to make sure there wasn't an air bubble trapped somewhere.