Once again, 88 TBI, EGR problem - poor performance, misfiring, etc.

At idle - no vac. At CRAWLING speeds, just the slightest amount of throttle, and I'm at 50-60 KPA (or between 15 and 18 inches). I'm not talking about cruising, I'm talking about just enough throttle tip in to get the vehicle moving at a slow pace.

Should I be seeing this vac at such low speeds from BEHIND the solenoid? In front of the solenoid I get about 60-70 KPA. If not, where would you go with this problem? Bad solenoid... 5 days old, or ECM grounding for no reason, or other?

 

Unplug the wiring connector and check again. My EGR opens with the slightest touch of throttle.

 

When the solenoid is unplugged, the vehicle runs like new. I haven't checked the vac under that circumstance since the problem only manifests itself when plugged in, but I will.

 

Get a new solenoid.

 

This is a new solenoid, but it could have been bad out of the box, so I'll try another Kepp you guys posted....

 

If it runs fine with the solenoid unplugged it couldn't be the solenoid. Sounds like your harness has a short to ground between the solenoid and the ECM keeping the solenoid in operation all the time. Disconnect the battery and do a continuity check between the black wire on the connector and ground. If it has continuity the wire is shorted to ground or possibly the ECM is bad.

 

Funny you say that....

I just went out with my multimeter... in P/N I don't have this problem, when in gear, and "Blipping" the throttle, all of a sudden there's 12v's at the EGR Sol. lead....

So the solenoid is good, it's just doing what the ECM is telling it too do. Now tracking down a bad ground will be a nightmere if that is indeed it. I just replaced the ECM about 45 days ago, if that.

 

If your only getting +12V when revving, but no voltage at idle, then the ecm is doing what its supposed to do. IIRC, the solenoid is controlled by one of the PWMs in the ecm, and not switched to ground like other stuff. Assuming you have the right solenoid and it seals, then it may be a problem with how the solenoid is hooked up vac. wise.

 

The solenoid is hooked to the same pink/blk wires that the injectors are and is hot at all times. The ground is through the ECM just as the injectors are.

 

Here's what I've determained to be true in my vehicle:

There is 12v at all times to the Red/pink lead. The ECM grounds the leads at the slightest increase in throttle - BUT - only while in gear. There is no continuity when going from the black lead to chasis ground, eliminating the idea of a constant/bad ground.

The TBI Vac setup is pretty simple, I've gone through all the lines, and everything is setup according to diagram, and the hoses are in good condition, without leaks.

The TPS shows .59v at idle, up to 4.31v at WOT, and smooth operation throughout that range.

The vac before the solenoid is about 60-70 KPA, and 55-60 behind the solenoid when grounded.


I guess it's a bad ECM ( ) but I don't see how, why, etc. I just don't understand - if the ecm was shot, wouldn't it be grounding the connection all the time, or none of the time... not just when it thinks it should? (All or nothing theory)


I guess I'll just have to bite the bullet and try yet another new ECM, but if that doesn't fix it, I'm gunna go buy a Ford.....








... no, I'm not that desperate.

 

The EGR in my stock 350 bin comes on at 3.12% TPS. That is the slightest movement of the throttle.

 

Guess I was wrong about the driver in the ecm. Anyhow, as long as it only comes on when the throttle is open the ecm is most likely fine.

 

Sounds like you have a bad egr valve to me. The solenoid is typically turned on as soon as the ecm sees tps movement. But the egr is supposed to sense exhaust backpressure and keep the valve from opening even when vacuum is applied. Once backpressure is correct then the valve will open. Look at your valve if it is original it should have a N stamped on it for negative backpressure. A test you can try if you have a hand held vacuum pump is to hook onto the valve with the pump and with the engine off pump it up the valve should open and hold vacuum. Then reach in and crank the car and the valve should slam shut and bleed the vacuum off before it even starts. If that does not happen then again the valve is bad. Don't buy an aftermarket egr valve either get it from the dealer, the aftermarket ones are junk and will give you the same problem.

Hope this helps.

 

Thanks that maybe what is wrong with mine. I did that check and it stayed open.

 

Again this test only works on negative backpressure valves with the N stamped on it.

 

I can't wait to try this, but a few questions -

Where would the N be stamped on the valve? On the base, or the top?

What is the difference between a negative backpressure valve and any other valve?

Do you have a part number for the N valve?

Is there any way of testing a normal valve?

Thanks, GOY

 

Looking down at the top of the valve you should see a bunch of numbers infact two rows of them if I remember right. After the second set of number the N is the last thing.
Something like this: XXXXXXXXXX
XXXXXXXXXX N

The other types of valves are positive back pressure and ported. There is no good test for a positive backpressure valve which will have a P instead of the N. The ported valves do not have anything stamped on it. So if there is no P or N then it is a ported valve. A ported valve will open with vacuum no matter what, if it has vacuum it is open. Any other questions let me know.

 

Now, this isn't so much about the direct situation at hand, but I want too make sure I have this right.

A negative BP valve requires no exhuast backpressure to operate then, right? So you could run open headers, and the thing would still stay shut until needed?

A positive backpressure valve works when there is substantial exhaust backpressure to keep it closed until needed then, right? So if one was to run open headers, or a really free flowing exhuast, they might have the problem I'm having - a valve opening before it should due to a total lack of back pressure.

 

It is not that simple unfortunatly. The negative and positive have to do with the pulsing inside the exahust system not so much that actual amount of backpressure. What you are asking about the header issue and such does not really work that way. But in a nutshell changing the exhaust can affect how the valve works you are correct.

 

Best question then is : Which is the best choice to use, for *As Near As Proper Function* as possible with a free flowing exhuast? That's taking into account that with a free flowing exhuast no valve will work within 100% factory specification.


Sorry for all the questions, but the more I know, the better I would be able to answer them for others in the future, per se' you weren't around

 

I have not personally tried switching around different types of valves to see if one would be better then the other for what you are asking to do. I would just stick with what it came with from the factory. The solenoid that controls the valve can be specific to what type of valve it has. So trying to mix and match would probably just cause more problems then it is worth.

 

I am experiencing the same problem with a freer flowing exhaust. I still have not been able to find a solution. I have tried a LS1 egr valve also which was made for less backpressure with no luck. I am going to make sure again that the solenoid is working and that there is enough vaccum again. What should the vaccum be at the egr port at idle?

 

I get 60-70'ish KPA BEFORE the solenoid, and 50-60'ish AFTER the solenoid when it's open....

I'm starting too wonder if changing the intake manifold may help the problem, as the EGR passages would be shaped slightly differently.

Either way, I'm being pushed further and further towards a TPI conversion if there is no short or simple answer forth coming, because this is unbareable and simply BUll$h1t

 

Simple solution: EGR blockoff plate and tune the egr out of the calibration. Probably not the one your looking for, though.

And if you think its hard now, wait till the egr is buried underneath the manifold like it is with tpi.

One thing that would be worth trying is testing teh solenoid with a hand-held vacuum pump while its in operation. You could immediatly rule it and the ecm out then. You should be able to get the solenoid to hold vacuum on the engine side for a reasonable ammount of time with the throttle closed. When open, it should let the vacuum bleed off.

 

I've worked on TPI cars - owned a few, and yes, the EGR is a pain to get too, but at least when it's in good working order, it stays shut when it should, as oppossed to being "Picky" about what kind of exhuast is around it. Love TBI, but hate it's EGR.

Above in my earlier posts, I've tested the ECM and Solenoid for proper operation. Both worked fine, as they should (new solenoid 2 weeks ago, new ECM about 60-90 days ago).

I can't beleive there isn't a more direct answer to this.

 

The worst thing the exhaust should do is cause the valve to remain closed due to low back pressure. Not open. The only way it would open with no vacuum is if excessive backpressure was present.

EGR on old vehicles is a ROYAL pain in the *** due to all teh accumulated deposits. The crap breaks off and causes all sorts of headaches when it enters/lodges in the valve. Had it happen with almost all my cars, no matter what the induction system was. These old V8s run real dirty. My present V8 produces a good deal of soot. Even alot of modern cars produce alot of soot and thats what causes the problems with egr valves. If you want REAL egr problems, try one of those crappy CPI systems that came on S10's. Constant leaking from that junk injector assembly causes MASSIVE ammounts of deposits that perpetually block the valve open.

I didnt see you mention replacing the valve. If you havnt done that, nows the time to do it. If a bleed oriface is plugged it could easily cause the valve to creep or remain open when the solenoid is closed. If the origional valve is there, then the easy solution is most likely to go down to the local chevy dealer and say "I have a blah blah blah and Id like to order a new EGR valve for it."