Poor MPG. Tune-up suggestions?

[Wajulia]

Remember that the dwell only fine tunes the idle mixture. I would not expect it to cause such a change in mileage. Again, that mileage is saying the carb is full rich at all time. I bet there is black smoke coming from the tailpipes when you accelerate. Have your wife follow you. If so, it is definitely not the dwell. But, the idle mixture solenoid fails with the carb full rich.

Did you hear the mixture solenoid click when you jumpered the wires?

There are other ways to check to see if the solenoid is operational. The easiest is to listen for the clicking, not conclusive because older ears don't work so good sometimes

You can place a coffee stirrer down the D" shaped hole on top of the carb and you should be able to see it dance, but I don't know how comfortable you would be sticking something down your carb

 

[22229stingray81]

Yes, I heard the mixture solenoid clatter when I jumpered, and as you suggested, I just looked down the carb and saw it clacking away. No black smoke coming out the back, either.



-Tatortot

 

[22229stingray81]

Well, I feel stupid. I had the dwell meter on 6-cylinder TACH, not dwell angle. So please ignore my above results. Man, so dumb.... In my defense I was miffed about the THUNK I heard while driving... still can't get over what it could be.

After stepping away from it for a while and having some dinner, I went out to the garage to go over everything I did, and that's when I noticed the meter on TACH. Anyways, I'll do the dwell measurement as soon as possible. Not sure the weather is going to cooperate tomorrow. Stay tuned, and thanks for all the replies so far.


-Tatortot

 

[GerryLP]

All very good checks, but it has been my experience with my L81 that poor gas mileage is the result of vacuum leaks. A stuck PCV valve, bad valve cover gaskets, carburetor leaks at the base, and such can cause vacuum leaks.

The fastest (although a bit messy) way to check for vacuum leaks is to spray these areas with WD-40 while the engine idles. Disconnect and cap the vacuum source to your headlights for the moment. Remove the bulky air cleaner. Visually check the vacuum hoses to the engine. Buy a fresh can of wd-40. start the engine and warm it until it gets to its curb idle speed. Systematically spray the wd-40 around the valve covers, carburetor, egr valve, etc).

If the RPM increases while you spray an area, it means a vacuum leak exist at that location. Messy, but perhaps will help you clean the motor as well. lol

If that does not reveal a vacuum leak, then apply vacuum to the headlights vacuum system and troubleshoot for vacuum leaks (usually, if the headlight doors are closed or open, the vacuum gage should reach and hold its maximum reading. If it doesn't, then you have a leak somewhere in the headlight vacuum system).

 

[22229stingray81]

As always, thanks for the advice, Gerry. I'm going to need a little further help with the vacuum check (I don't have a vacuum pump or gauge) at the headlights, but I do understand what you're talking about. And stuck PVC, or EGR sounds very plausible here. Ya know, years back I had all the vacuum lines gone through and fixed the leaks I had, but, that was years back, and we all know what can happen when you let a 'vette sit So I think you may be on to something. Hopefully I'll get to working on it tomorrow. Should be a nice day.

And I resemble the remark about a clean engine! It takes YEARS to create such a patina, Gerry!

-Tatortot

 

[GerryLP]

As always, thanks for the advice, Gerry. I'm going to need a little further help with the vacuum check (I don't have a vacuum pump or gauge) at the headlights, but I do understand what you're talking about. And stuck PVC, or EGR sounds very plausible here. Ya know, years back I had all the vacuum lines gone through and fixed the leaks I had, but, that was years back, and we all know what can happen when you let a 'vette sit So I think you may be on to something. Hopefully I'll get to working on it tomorrow. Should be a nice day. And I resemble the remark about a clean engine! It takes YEARS to create such a patina, Gerry!-Tatortot

I forgot to mention that if you are spraying in between the intake manifold and the cylinder heads, then that would likely be an intake manifold leak. Good luck, buddy.

 

[22229stingray81]

Gerry, we're getting somewhere. I couldn't wait 'til tomorrow and poked around the engine bay these evening, and look what I found:

There's a broken vacuum line on the passenger side engine bay. It is in front of the red EGR solenoid connector. Looks to be going to the right into a 3 way red "T" connector. I tried to pull off the other end, but it's so perished that it's just ripping. So, how much do you think this plays into my problem? Also, how "stiff" is the EGR valve to move? Mine is unlike the one pictured in the shop manual; it only has a few small holes on the back, rather than wide-open spaces. Anyways, I got in there with a 90 degree "dentist" pick tool, and tried to push up the valve and it wouldn't move. Are they difficult to move?
Okay, last, and total newb question..... how do I buy vacuum hose? I assume I just go to the parts store with the piece I need in hand, is that good enough? Or do I need a specific "type" for this application? And for future reference, how is the diameter measured, inner opening, or outer?

While I was in there I also checked the PCV valve. It "rattles" like the shop manual says it should. I'll do the other on-car checks when I start her up tomorrow. Anyways, thanks all for helping me in my saga.

-Tatortot

 

[bill81vette]

try Dr rebuild..in ct.he has pretty good stuff when it come to the vacuum
hoses
Hard to believe the hoses would go bad....
hopefully you found the problem!!!

 

[22229stingray81]

Okay, strap in and pack a lunch:

Went to Autozone and got 4 feet of the closest thing they had to the old vacuum line. It seemed to be just a TINY bit larger inner diameter. Anyways, got it connected to the EGR solenoid and to the red T fitting. Then I went for a cruise. It seriously seemed to idle better. Then I get home, and I believe it is stumbling again. Just 50-100 rpm drop at random intervals. Engine shakes a good bit when this happens, too. I hooked up the dwell meter with and without air cleaner. FWIW, I did not plug that tube that comes off of the air cleaner and into the passenger side valve cover. I did put my hand over it to see if the dwell changed, and it didn't. Anyways, dwell reads a steady 2.8-2.9 degrees. Not sure what that means.

Okay, now we get weird again. With the air cleaner on, the dwell jumps all around. I did this test twice as the leads to the dwell meter kept coming off. Outside, it was reading between 29 degrees and 36, jumping all around. Then I brought it into the garage and did it again, this time with it much lower; around 21 degrees and jumping up to 29 degrees. When I revved the engine, you can see it jumps up into the 30's. So, what the hell does this mean?

Click here for youtube video of dwell meter.

Lastly, I had a small mirror behind the EGR valve and I cannot see the diaphragm move. Ya think the EGR is worth replacing? Like I said, in the past the EGR has caused me lots of problems... engine vibration and whatnot. Looks like somebody used red sealant on there... gonna be hell to try and get it off. Need to pick up some sort of socket elbow too as I can't get a socket past the valve itself. I also checked the PCV valve and it seems to be working. I can feel vacuum and it "rattles" as per the shop manual. Anyways, advice is appreciated, thanks!


-Tatortot


Edit: And if anybody wants to watch 2:30 of my 'vette idling in and out of gear, click here. Maybe you can see the RPM drops.

 

[22229stingray81]

Update: I don't think I'm getting vacuum to the EGR. Okay, I know I'm not getting vacuum to the EGR. I pulled the vac line from the EGR and I can't feel any suction. I trace it all the way to the front of the carb, pull off the vac line from the carb, and I don't feel any suction from there either.

I just read in the manual that "During cold engine operation, a signal from the ECM energies the EGR solenoid blocking vacuum to the EGR valve." So perhaps my engine wasn't warm enough (it had been sitting for a while). It just started raining so I can't go out for another spin to bring temps up again. Ugh.

I also suspect an issue with the EGR vacuum solenoid; at least that's what I think it's called. Please see pic below.

There are these 4 inlets/outlets next to electrical connectors The bottom most one, circled red in the above pic, appears to be broken at the front end where I put a line through it, but the back is still attached. I'm not sure what the function of it is, but it seems like it isn't attached to anything and is just terminated there. I can't see how the broken front part impacts anything, but what is the function of that connector? MAN a replacement is expensive.

-Tatortot

 

[22229stingray81]

Another nice day, more wrenching..... Here's what I've found so far:

1. Dwell meter stays at 2.9 degrees, BUT when the engine runs rough and drops by ~100 rpm, it spikes to ~30 degrees. It jumps all around, up to 36, down to 27. Then, when the engine smooths out, back to a steady 2.9 degrees.

2. The EGR diaphragm does not appear to be moving. Can someone please confirm that the diaphragm in the picture below (where the red arrow is pointing) is supposed to move up and down? I can't even really push it with a right-angle pick tool. As per the shop manual, I AM getting vacuum on the line feeding into the EGR valve when I rev the engine.

Any thoughts on what's going on here?

Gerry, I did spray WD-40 around the valve covers, and carb base, and didn't detect any change. I wimped out a bit as I didn't want to spray the stuff where I couldn't reach it.

-Tatortot

 

[GerryLP]

Another nice day, more wrenching..... Here's what I've found so far:

1. Dwell meter stays at 2.9 degrees, BUT when the engine runs rough and drops by ~100 rpm, it spikes to ~30 degrees. It jumps all around, up to 36, down to 27. Then, when the engine smooths out, back to a steady 2.9 degrees.

2. There is no vacuum going to the EGR valve. If I trace the vac line down stream, it plugs into the front of the carb. When I remove the line, there is no suction coming out of the carb. It is the solid purple line coming off the EGR solenoid to the front of the carb in the attached picture of the emissions label. Should I be feeling vacuum here?

Any thoughts on what's going on here?

Gerry, I did spray WD-40 around the valve covers, and carb base, and didn't detect any change. I wimped out a bit as I didn't want to spray the stuff where I couldn't reach it.

-Tatortot

tattor,

The EGR valve is inop at idle and at deceleration (the computer detects these engine regimes, and turns-on/off, vacuum to the EGR through the solenoid), so don't let that detract you.

Honestly, it is a pain, but it will be worth it when your Vette starts running great. Vacuum being utilized/bled/leaked at idle causes all kinds maladies.

1) Check all your emission's hoses to make sure they are routed to where they should be. That emissions label is great for that.

2) with a hand-operated vacuum pump, disconnect the hose at the EGR and connect the hose from the vacuum pump. What you are doing is checking for movement of the diaphragm. This movement represents the movement of the needle valve inside, and it will show if it stuck open (it is spring-operated closed). It being stuck open, it would explain the rough operation at idle.

3) Next, if the EGR checked good, then check that the gasket is in good condition. Check that the bolts are not loose. If everything is fine, then let's move on to the next set of checks.

4) use vacuum pump to check the thermal vacuum switches (TVS). If they are stuck open and the engine is cold, you will not be able to build a vacuum signal on the pump's gauge. At this time, it really does not matter which port on the TVS to which you hook-up the vacuum pump because we are checking whether it is stuck open or not. To determine that the TVS is working correctly, the vacuum pump should build-up a vacuum signal on the gauge with the engine cold. If it is not, then it means that they are bleeding vacuum signal at a time when is not needed or it is detrimental to engine operation.

Note: Should the TVS be stuck closed when engine is warm, this would not necessarily explain the rough operation at idle. It would most likely explain emissions test failure.

5) prepare for engine start. With a couple of segments of rubber tubing the correct size and matching vacuum T-fitting (auto Parts store's "HELP!" rack carries a sorted kit. What you what to do is to reconnect the EGR's hose at the EGR and disconnect the same hose at the solenoid (the other end). At the solenoid end, install a short segment of hose to the solenoid, a similar diameter size plastic T fitting, and another small segment of hose to the opposite plastic fitting tube. Then, connect the remaining end on plastic fitting to the vacuum pump hose or vacuum gauge. In effect, this will help you tap into the vacuum signal that may be present at the EGR valve hose, in order to determine if there is a vacuum signal present.

Note: There is supposed to be a vacuum signal at the opposite side of the solenoid. The computer determines when the solenoid is activated to apply vacuum to EGR valve. By patching the vacuum pump or vacuum gauge like I explained above, will help determine if the solenoid failed and is allowing vacuum signal to the EGR during engine start or engine at idle speed.

6) Start engine and while it warm's up, check the vacuum gauge. There should be no vacuum signal when engine is warm.

[i'll start my next posting at step "7)" to continue with troubleshooting.]

 

[22229stingray81]

Wow Gerry..... helluva trouble-shooting guide; thanks! I'm going to pick up a vacuum pump ASAP. Also, I edited the post you quoted; I am indeed getting vacuum to the EGR (you have to rev the engine, as the shop manual clued me in to, and you confirmed). The EGR valve does not appear to be moving at all, so I guess I gotta pick up an EGR as well. And some sort of flexible elbow to get to the bolts. They are TIGHT on there.


-Tatortot

 

[GerryLP]

Tattor,

I am also not very worried about your RPM gauge dropping 200 RPM's. The tach in the Vette is not designed for accurately adjusting an engine. It lags a bit behind of what really is going on in small increments of engines speeds to enhance its accuracy during large increments.

Also, the dwell is supposed to flux (on an analog dwell gauge, the movement of the needle is called,"hunting"). So 27 degrees to 36 is normal. A steady value means open loop.

 

[22229stingray81]

You are the MAN, Gerry.

I'm still slightly worried about the RPM change though, as the idle becomes rough when this happens. I wonder if it becomes rough in "closed loop" operation, as that is when the dwell meter starts to 'flux', and not stay at a fixed 2.9 degree point. Hmmmm.

I suspect the EGR. Like I said before, the EGR caused some SERIOUS shaking about 10 years ago, and it was replaced. I don't see why it can't happen again.

 

[GerryLP]

Once you are able to determine that the EGR valve is not leaking and operating correctly, I would recommend we look at timing, engine curb idle speed and such. To make sure engine is at the right point to let the closed loop system do its thing.

 

[GerryLP]

You are the MAN, Gerry.

I'm still slightly worried about the RPM change though, as the idle becomes rough when this happens. I wonder if it becomes rough in "closed loop" operation, as that is when the dwell meter starts to 'flux', and not stay at a fixed 2.9 degree point. Hmmmm.

I suspect the EGR. Like I said before, the EGR caused some SERIOUS shaking about 10 years ago, and it was replaced. I don't see why it can't happen again.

You may be right. Heck, a broken diaphragm on the EGR valve, in effect, is bleeding vacuum signal, so once you can declare the EGR is or is not the problem, that will determine our next step.

Since timing, curb idle, and the rest of the adjustment affect the engine, we'll have you set timing to what it is supposed to be, and that will probably cause backing out the curb idle adjustment, which is linked to throttle position sensor. So, it could be causing the dwell problem.

Did you clean or replaced PCV valve already?!?!

 

[22229stingray81]

You may be right. Heck, a broken diaphragm on the EGR valve, in effect, is bleeding vacuum signal, so once you can declare the EGR is or is not the problem, that will determine our next step.

Since timing, curb idle, and the rest of the adjustment affect the engine, we'll have you set timing to what it is supposed to be, and that will probably cause backing out the curb idle adjustment, which is linked to throttle position sensor. So, it could be causing the dwell problem.

Did you clean or replaced PCV valve already?!?!

Yes, the PCV valve has been cleaned, and thoroughly tested. I can feel the vacuum when I disconnect it, and it "rattles" when it is off the car, per the shop manual. But, it's only $4, so why not replace it any way, right?

Okay, I'm ordering a vacuum pump, new EGR (I'm 95% sure the diaphragm isn't moving) with gasket and will report back. Thanks again.

-Tatortot

 

[GerryLP]

Gerry, we're getting somewhere. I couldn't wait 'til tomorrow and poked around the engine bay these evening, and look what I found:

There's a broken vacuum line on the passenger side engine bay. It is in front of the red EGR solenoid connector. Looks to be going to the right into a 3 way red "T" connector. I tried to pull off the other end, but it's so perished that it's just ripping. So, how much do you think this plays into my problem? Also, how "stiff" is the EGR valve to move? Mine is unlike the one pictured in the shop manual; it only has a few small holes on the back, rather than wide-open spaces. Anyways, I got in there with a 90 degree "dentist" pick tool, and tried to push up the valve and it wouldn't move. Are they difficult to move?
Okay, last, and total newb question..... how do I buy vacuum hose? I assume I just go to the parts store with the piece I need in hand, is that good enough? Or do I need a specific "type" for this application? And for future reference, how is the diameter measured, inner opening, or outer?

While I was in there I also checked the PCV valve. It "rattles" like the shop manual says it should. I'll do the other on-car checks when I start her up tomorrow. Anyways, thanks all for helping me in my saga.

-Tatortot

Tator,

i am am sorry for not examining (only looking) at these picture before. But the hoses for your EGR need replacement. Remember, vacuum signal is always present on to these hoses. So you are driving around with a vacuum leak. That will drive fuel consumption.

Now, the two hose connections with the red plastic "Y" fitting are connected as follows; the straight fitting on the side of the red "Y" fitting where the two legs of the "Y" are, gets connected to the top port of the white solenoid (the solenoid underneath the red one). The other leg if the red "Y" fitting on the pair side (the red fitting leg at an angle from the straight fitting) gets connected to the bottom port of the red solenoid. The single red fitting end, gets connected by a hose to the front-passenger side vacuum connector at the base of the carburetor. That means that all three red fitting ends have vacuum present all the time the engine is running. If those are are broken or cracked, then you will have a significant vacuum leak to cause an increase in gas consumption.

Here is the picture in Gail's engine

The hoses with green and red stripe are just what I explained above. They correspond to the emissions' label purple and the small portion that is identified as "RESTA". The screw driver in my picture is pointing to the bottom solenoid bottom port. This is the one that connects to the EGR valve.

The hose at top port of red solenoid connects to the carb's top rear tube.

WARNING!!!!!!
WARNING!!!!

These solenoid ports are plastic, and given their exposure to heat over the years, they have become very brittle.

SOAK THESE HOSE CONNECTIONS WITH WD-40 BEFORE ATTEMPTING TO DISCONNECT THE ASSOCIATED HOSES!!!!

After soaking for a 1/2 hour, rotate the hoses gently to make them loosen their grip upon the plastic ports. Once you get them to rotate, spray some more WD-40 on them, and GENTLY pull the hose segments straight out (without forcing them laterally or sideways). You can wiggle them gently, just make sure you are not forcing the plastic nipple of the solenoid ports to lateral movement.

A few years ago, I broke the red solenoid on my Vette. ugh It took me a while to find a replacement:cry. It cost me dearly.hbthumb

This should fix your fuel consumption problem. Run the dwell gauge

 

[22229stingray81]

Gerry,

That vacuum hose has been replaced, as well as any others that looked cracked or dodgy. I even traced the entire lengths of the headlight hoses and couldn't find any problems, though I still realize there could be a leak. So, that picture was before the hoses had been changed. Still getting an intermittent rough idle, followed by ~100 rpm drop. I ordered a new EGR valve, so we'll see if that's the problem soon enough. Also ordered a vacuum pump so I can do the more in-depth vacuum tests. Thanks for all your help!

-Tatortot