EGR question.

[muncher]

Hi All.

I have a 1987 GMC Sierra with a 305 tbi engine. I have just completed a complete engine rebuild with some minor performance enhancements. Engine sounds sweet!!

Whilst doing the rebuild I removed the entire EGR system. Don't need it here. As you would expect the yellow service light comes on. Other than the light, the truck drives real nice. No idle problems or anything other than the light.

Do you have any ideas on how to sort the yellow service light issue other than removing the bulb.

Cheers Guys.

[Liberty]

I guess there might be a resistor you could throw in there that will trick the light.

[muncher]

I have read somewhere about fitting a resistor. Where would I put it? Cheers

[geezer#99]

Easy peasy!!
A small piece of black tape makes it disappear.

[Zane M]

I don't know anything about the resistor fix, but if you can find a tuner that can/ will burn a OBD1 chip, you can get the EGR deleted from the "tune".

[muncher]

I will look in to getting a tune done. There was an O2 sensor in one of the old exhaust manifolds. With the new Headman Headers I fitted the sensor is no longer used. My guess that's what's causing the light on. Any other way round that, other than tune?

[Liberty]

Did 87 have OBD1? Or is it kind of just a "dummy" light in the literal sense? I'm thinking the CEL is in the same spot the Choke light would be, in which case the Choke light looked for voltage or something.

[muncher]

I have it booked in to a Chev/GMC specialist next week. From what he says it just needs reprogramming to ignore the O2/EGR system I removed. He will also give it a tune.

[hatzie]

Originally Posted by muncher

I have it booked in to a Chev/GMC specialist next week. From what he says it just needs reprogramming to ignore the O2/EGR system I removed. He will also give it a tune.

That's the best way to do it.

[slotard]

I'd expect an O2 sensor to be helpful but I'm talking out my ass.

Hondas with CELs thatre pre OBD1 are usually referred to as OBD0 but I don't know about GM.

[hatzie]

1987 TBI uses the GM proprietary Assembly Line Data Link ALDL interface at 160 baud on Terminal D or E. I don't believe the early TBI trucks had 8192 baud data on terminal M. Look... if terminal M is populated then it probably has 1892 baud data.

GM OBD1 is 8192 baud ALDL through a J1962 OBD2 style connector under the dash.

The O2 sensor is actually useful and can yield better fuel mileage but it can be run in open loop from a fuel map.

[SkinnyG]

Check the code first, and find out what it is unhappy about.

When I put oversized exhaust on a Nissan Hardbody, the factory computer as unhappy about it and threw a code blaming the EGR. I had functioning EGR, it just wasn't happy about something.

In the case of the Nissan, all it did was look to see if it had the right temperature in the intake manifold. This told it that the EGR was working. Since I had opened up the exhaust, the intake just didn't get as hot. I found the thermistor that the ECU was checking, and replaced it with the resistance that the ECU wanted to see.

I don't know GM computers, but....

1. Find out what the code actually is.

2. Find out how the faulty system works.

3. Repair it or fake it as necessary.

[kalbert]

You most definitely want the O2 sensor. It is not an "emission" thing. It's a fuel mixture tuning thing. You want the ECM to have more data available to it and more control over the mixture, not less. Purely *open* loop EFI systems are very poor performing all around.

EGR doesn't hurt anything. A properly functioning EGR system is undetectable and doesn't have any impact on power output at times when you ask the engine for high output.

I would start by doing the obvious, install the O2 sensor (and strongly recommend the EGR). If you are running a thermostat rated at less than 170° swap it out for a hotter one. If you really must run a 170° or less you'll need a custom PROM with fuel tables calibrated for the lower temp and the closed loop criteria modified. If your cam has more duration than 216° you'll also need a custom PROM with fuel map calibrated for the lower engine vacuum. Run the engine at idle, set the timing, and check vacuum. If you've got less than 18" at idle your cam has too much overlap and you'll need a custom PROM to accommodate it, most of the factory tunes can handle down to 18" ok. If you have 18" or more you should be good to proceed running the engine until it is operating temp, and with all the necessary equipment connected, should not have any CEL. If you do have CEL, determine the diagnostic code that is set and proceed accordingly...

[Jrgunn5150]

Originally Posted by kalbert

You most definitely want the O2 sensor. It is not an "emission" thing. It's a fuel mixture tuning thing. You want the ECM to have more data available to it and more control over the mixture, not less. Purely *open* loop EFI systems are very poor performing all around.

EGR doesn't hurt anything. A properly functioning EGR system is undetectable and doesn't have any impact on power output at times when you ask the engine for high output.

I would start by doing the obvious, install the O2 sensor (and strongly recommend the EGR). If you are running a thermostat rated at less than 170° swap it out for a hotter one. If you really must run a 170° or less you'll need a custom PROM with fuel tables calibrated for the lower temp and the closed loop criteria modified. If your cam has more duration than 216° you'll also need a custom PROM with fuel map calibrated for the lower engine vacuum. Run the engine at idle, set the timing, and check vacuum. If you've got less than 18" at idle your cam has too much overlap and you'll need a custom PROM to accommodate it, most of the factory tunes can handle down to 18" ok. If you have 18" or more you should be good to proceed running the engine until it is operating temp, and with all the necessary equipment connected, should not have any CEL. If you do have CEL, determine the diagnostic code that is set and proceed accordingly...

^^^ This, if you have removed the rear O2, no real harm, although you can trick the ECU without a tune by using a spark plug antifouler.

If you removed the front O2, you're running pig rich all the time and need ot deal with that.

Blocking, or removing the EGR will not trip a light on a OBD1 Chevy, in my experience. I used to block them with golf tees when they got stuck.

[slotard]

"Closed loop" means there's a complete feedback loop, so it's using the O2 sensor. Open loop means there's a break in the feedback loop, not using the O2 sensor.

There are decent open loop systems too, one of my motorcycles is EFI and has never had an O2 sensor.

[kalbert]

Whoops, you're right. I meant to say open loop there, not closed!

[muncher]

Thanks for all your in put. Loads of great info there. I removed the EGR/emission gear to clean up the engine bay and also hope to get it to breath easier. With the new headers fitted the O2 sensor was removed. Its all replaceable but would like to leave with out if I can. Truck runs sweet, so smooth. The tune will give me a better idea what's going on. I will post results when done

[jjmorrse]

At some point, in the search for eliminating parts, it might make sense to just get a carburetor and a regulator, since a major benefit of an EFI is that it can compensate for the data it gets from the engine. Then the fix for the light is to remove the bulb. Then your fuel map consists of a screwdriver, a vacuum gauge, and a timing light.