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'79 DeVille fluctuating performance from trip-to-trip (very strange!)

Started by Benzilla, October 12, 2013, 04:53:30 PM

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Benzilla

I've been having a strange issue with my '79 DeVille Phaeton. Most of the time, it has a predictable amount of throttle response, however I've been noticing for quite some time now the following issue:

In the course of a day, if I take the car for a long enough drive to reach & hold full operating temp for a while, then drive home & park it for a while (maybe a few hours), when I go back out for the second drive of the day, the car has a VERY noticeable power increase for a while after startup.

It's as if my throttle input is being amplified, and the car feels extremely responsive & peppy. If I keep driving, this phenomenon slowly goes away within about 5-10 minutes of driving, and I'm left with the performance level that I've come to expect as normal.  It's like while the power influx is happening, the engine isn't working nearly as hard to accelerate, and there's a big difference in that "butt-to-the-seat" feeling of propulsion.

Facts:

I don't get any kind of power flux during the 1st trip of the day, ever. It's only after it's been heated up fully, and cooled down to some extent.

This problem has transcended a carburetor swap, and full ignition system replacement. Meaning that unless the carb I rebuilt has the same theoretical problem that I'm somehow overlooking; carb adjustments make no difference in this erratic performance. Choke, linkage & all other variables have been adjusted according to the factory service manual.

The car still wears what I believe to be the original catalytic converter, which makes me wonder about heat sensitive back-pressure fluctuation of some kind?

The engine has 234,000 miles on it, with no known complete rebuild. Could this be some kind of compression issue or other internal disturbance?

I've had a long running off & on battle with detonation. I think I have it solved, but that is to point out that the detonation would also come and go seemingly randomly. However, the pinging always seemed to vanish while the car was having a "power influx"

____________

That's all I have for now, I'm left scratching my head. Soon I will post a complete project thread with the car's full history in the restoration section, but for now I just had to get some light shed on this.

Thanks for looking, I can't wait to hear some ideas!



TJ Hopland

What components were replaced as part of the 'complete ignition' replacement?

Where is the vacuum advance vacuum line connected on this car?   Is it direct to manifold or to carb or through a thermal switch?

Does this car have a EFE?  That's the 'heat riser' or 'exhaust flapper'.  I believe if its got one it will be between the right exhaust manifold and the down pipe.   Could be hard to see from above in the RWD cars.  A clue would be a metal vacuum line going down in that area.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Benzilla

By complete ignition system, I meant new distributor base (federal version, bypassing the "Electronic Spark Selection" or "ESS" box from CA) & vacuum can; cap; rotor; ignition coil; spark plugs; plug wires; starter. all within the last 18 months.

I didn't think to mention, the vacuum advance is run direct manifold vacuum. This is a change I made. The car was originally from California, with all the special equipment, so the vac advance ran through a thermal vacuum switch (TVS), disabling cold operation advance. The same TVS also cut cold operation of the EGR valve. I eliminated the TVS entirely & ran the EGR valve direct from the carb, as the federal versions were routed.

Yes, it came with an EFE valve, which is actually a sore spot for me.. You see, when I bought the car, someone had already installed a custom temp gauge under the dash, and in connecting the sensor for it; removed the TVS that was in control of opening the exhaust back up when the car was warm, and routed the hoses so that the system was always drawn closed by vacuum! Needless to say, I had a hell of a time figuring out why it always wanted to run hot until I discovered that. I bypassed the system entirely. However it has been brought to my attention that even though I've bypassed it's vacuum source, it could still be stuck in the intermediate / closed position by exhaust byproduct buildup. I admit, I have yet to check it's position since I'd need a lift or some ramps, which I am temporarily without. 

I should also note, though; that the car has never actually overheated to the point of "boiling over" in the 7 years I've owned it. Also, I did a coolant flush, new heater core & thermostat last winter, for what it's worth.

TJ Hopland

You have either dealt with or are at least aware of all the typical stuff.  How about the cat converter?  Is it still there?  Is it original?
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

CEC #20099

EGR Valve. Normally later 1970`s GM engines ran EGR thru a thermal switch. Cold engine - no EGR. Warm engine, EGR operates. EGR has a big effect on detonation. It reduces it. I would definitely replace the EGR valve with quality new item & check all passages.

Early GM pellet cat converters were a failure waiting to happen. Pellets melted & plugged it. GM did some recalls, if you screamed. If it`s orig. get it out of there. Go from there.

c chleboun #20099

Benzilla

As briefly mentioned in the original post, I believe it might still has it's original catalytic converter. If it has been replaced, it's at least the correct style & looks original. This car has almost zero rust on the chassis, so it's hard to date how old it might be. How can I find out if it's ruined without just replacing it?

MR. Cheleboun; See 2nd paragraph of reply #2 about the EGR valve routing. Forgot to mention, but I also removed the EGR valve, cleaned it, tested it's vacuum hold & movement, re-installed & checked for on-car operation a few thousand miles ago.  :)  Thanks for the notes on the converter!!

So, anyway... I made a discovery earlier. I was out re-checking vacuum lines for leaks, because I had this idea that perhaps an intermittent vacuum leak was causing engine vacuum at idle to fall lower than that required for the vacuum advance can to properly seat all the way, & throwing off the advance amount. In the process of testing this theory, I discovered that my 8-month old distributor's vacuum-can has, at some point in the recent past, gone belly-up, and is now a giant vacuum leak. Now... Though this is not the main culprit for the situation at hand, it might explain a dieseling (run-on) problem I've had the last couple of weeks, assuming the vacuum advance broke at the same time the dieseling began.

In light of this discovery, I've been trying to determine what vacuum advance I should replace it with. I've heard that most generic non-adjustable replacements pull too much advance, around 22 degrees, where as my service manual says the original only pulled 16 degrees on federal cars, less on CA. However I've also heard that the adjustable ones have to be adjusted to extremes to get the advance curve close to GM's original, because they pull full vacuum too soon, at too few hg, possible causing part-throttle-detonation. I have no proof of either of these statements. For now, I'll see if the advance-can on my original distributor is still holding air, and if so; put that on until I choose a replacement.

Scot Minesinger

Take a look at the EGR.  This often impacts performance the way you described.  Recommend it be replaced, as it has a rubber diaphragm and it is 34 years old.  This is not difficult to do, and inexpensive.  What it really is going is the car performs as normal after the first drive and is fouled on the first drive.  The EGR only influences performance after the car has driven for a good 30 to 45 minutes.  This same circumstance happened to me when back in the 80's.  No one could figure it out and I purchased the shop manual and it boiled down to the EGR or rods, so replaced the EGR and the car was like new again (partially because many other things were repaired trying to find this this problem).
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

bcroe

I had a 79 with detonation problems; tried everything, even water injection.  I finally
changed the timing chain set, and the problem mostly went away.  Since then, I have
realized the original timing set will always fail due to either years or miles.  Check yours. 
Bruce Roe

Benzilla

EGR VALVE NOTES..... See Reply #2. And again in Reply #5. 2nd paragraph of both posts. :)


Scot, I like your idea, but the car goes back to the diminished power after about 5-10 minutes of driving on the second trip. If it was the EGR coming unfouled, I'd think it would stay unfouled longer than that..?


Scot Minesinger

That is the problem with EGR valve, not consistent.  The valve is not expensive and made with a rubber piece.  It should be replaced anyway at a minimum for precautionary purposes.  This lost power and weak performance especially up a slight grade make driving unpleasant.  Replace the EGR and enjoy your Cadillac.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

"Cadillac Kid" Greg Surfas 15364

Ben,
Something to check is the mechanical advance in the distributor.  These HEI's were prone to "gunking" up the shafts and having the mechaqnicaql advance stick.  It could be that when the car is cold, the advance works correctly but when warmed up it sticks.  Run the car as per your normal "first trip", let it sit for one half hour then pull the distributor cap, pull the rotor and see if the mechanical mechanism is free.  I've seen these stick on relative "newly rebuilt" distributors, so it might be worth a try.
When your pulled the ESS, it might also be a good idea to verify that ALL the wires once going to and from the box are accounted for and not in a position where they could ground. 
I just re read the posts and if I am correct, you never changed the HEI from the original tied into the ESS.  That may be part of this issue.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

Benzilla

Thanks for all the advice!

Scot, though I've checked the EGR itself for proper vacuum holding, I can not say that you are wrong either. I think I'll order a replacement AC-Delco EGR valve & gasket to rule that out if nothing else. I can always keep the old one as a spare if it turns out not to be causing the issue.

Turns out my car's original 34 year old vacuum advance still works flawlessly & will hold a vacuum signal for days, which really brings shame on the 8-month old one that just broke... It seems like parts just aren't made like they used to be. Two months after I initially installed the new dizzy, the control module went out & left me stranded, turned out they didn't even have the heat grease applied under it sufficiently. Anyway, original vacuum advance back on until I decide on an adjustable one or not.

I took it for a drive with the original vacuum-can back on & working. It's definitely not a contributing factor to the current problem, as it still felt a bit sluggish. However, my idle is back to being smooth, and no dieseling yet, thank God!

-Ben

TJ Hopland

 I'm not sure I would be thinking egr, I would think it would have to be stuck pretty far open to make that big a difference in the feel of the performance and then if it was sticking that far I would think you would sometimes stall and have a really bad idle.

If you disconnect and plug the vac line to the egr does it still do it?    Disconnecting it for testing I would think would eliminate the possibility that its intermittently not sealing or opening too soon.   Just listen for detonation since it will be more prone to it with it disconnected.

How about something fuel pump or fuel tank related?   Like vacuum building up in the tank?

How about maybe the vacuum modulator in the transmission changing the shifting and that is what you feel?

I would not think it would make a big difference with that engine but how about the AC compressor running sometimes and other times not?
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Benzilla

TJ, I think you may really be onto something with the transmission! I'm trying to page through the service manual & get a better understanding of the system, but it does seem to coincide with low engine vacuum, especially in the space of time I'm pretty sure the vacuum advance was dead for (big vacuum leak). When I go back out for the second drive, I usually knock it out if high-idle prematurely in my rush to leave, and the idle would dip way down & get shaky... accompanied by the performance surge. Then, as the engine warmed back up to operating temp, the acceleration feel would go back to normal. I'm pretty sure you have a better grasp on this particular subject (trans) than I do, so does that information help any? It's just a hypothesis, but it makes sense if I'm understanding it correctly.

But... it doesn't make the car shift weird or anything.. I actually wish it drove like that all the time. It's just like it woke up!

TJ Hopland

I'm really not an expert on transmissions.   Just going through possible things that would effect the feel of the car.  I was just thinking that maybe it normally shifts too soon so that is why the flat performance.   Going from a newer 4 or more speed car to one of these 3 speeds can throw off our perceptions of what is right.

How long between these drives?   Seems strange that after a few hours it would be on high idle again.   That implies that the choke has closed again.  Also if it got to high idle means you had to step on the gas to get it started. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Scot Minesinger

All things rubber and original to a 79 should be replaced: EGR, vacuum actuator on trans, and distributor timing actuator.  Once you fix one thing another rubber part will let you down.  It takes more time trying to figure out which rubber part is bad, than to just replace them all straight away.  For example the vacuum modulator on trans is $14 at NAPA (just bought one a month ago to replace an original on a 68!) 
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

TJ Hopland

One problem I am seeing more and more when dealing with 70's and 80's car parts is that it seems very few parts are currently being manufactured.  I think in many cases we are getting NOS stuff, not necessarily factory old stock just not current production.  NOS may be good for some parts but stuff but rubber parts or parts with lubrication in them can be not so good.  I am sometimes amazed how dirty and beat the boxes are for some of the parts.   If the parts are new they tend to be what we now call 'imported' which tend to be pretty low quality at best.   

So the moral of the story here is just because you replaced it does not mean you can eliminate that as the cause of the problem.   Just look at the start of this thread, that happened to Ben.   We currently have other threads with bad condensers.   
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Benzilla

It's usually about 3-6 hours, I suppose. Really it's just whenever it's cooled down enough that hitting the gas will close the choke, but if I immediately kick it down from high idle, the engine idles a bit shaky for a minute. This was obviously most noticeable when I had vacuum leaks or my idle was set too low. My old carburetor had a lot of issues, including a sticky choke, so I got in the habit of flicking the gas before I tried to start it. I guess I often underestimate the time it takes the engine to cool down & actually need priming again.

I've replaced most rubber parts, and vacuum lines. I still need to get cruise-control working again, but the system doesn't seem to be causing a measurable vacuum leak that I can find. I'm positive the EGR valve has been replaced at some point in the car's history. It came off too easily, and was too clean inside to be original. I won't rule it out though.

Trans was rebuilt at the end of 2010 by a shop. Would they have replaced the vacuum actuator? I'll have to take a look around down there when I have a lift or some ramps around.

It really just feels like someone has swapped my rear axle between drives, from at 2:28 to a 3:08 or something. Then it fades back, lol. Keep in mind, this only happens if the event timing is just right, and I happen to start it during that "sweet spot" whatever it may be. It's not like this just happens every time I drive the car for a second trip in a day. I don't notice any increase in engine braking, either. It's not like when I manually ride 2nd gear out longer or anything. So odd...

-Ben