When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
The vacuum advance replacement basically went well but I opened up a can of worms. At this time I still haven't gotten all the worms back into the can. Several problems cropped-up shortly after the repair:
- Apparent failure of TPS, as evidenced by the engine responding to a gentle jiggling of the TPS wires
- Repeated engine stall when driving
- Idle misfire as evidenced by a subtle bump-bump-bump noise from the tail pipes
-Unexplained higher-then-normal idle speed
My first assumption is that these problem were necessarily my fault, the result of the fiddle-factor. That is, inadvertent damage to things during the course of other work.
I'll discuss these one-by-one
TPS failure
This is the aftermarket replacement for the old Bournes (black) TPS, installed 2-3 years ago. The attraction to this part it that it doesn't require the ridiculously overpriced adapter kit required to use the updated (red, smaller) TPS. However, I cannot recommend it. Years ago I used one on my XJS and it, too, failed prematurely.
(The reason I have the aftermarket Bournes-style is because the aftermarket (not OEM Ford) Mustang TPS that I was using failed awhile back. I don't want to go too far off into the weeds but the Mustang TPS is an iffy-proposition. I used one on my XJS and was very happy and have recommended it to others....but there is a catch. It is designed to operate from .5 to 5.0 volts. Whether or not it is possible to achieve the desired .32-.36 volt idle setting is totally hit-n-miss as it isn't designed to reliably operate in that range. I will also say that, from the driver's seat, the engine doesn't object to .38-.50 volts. It'll run perfectly happily at that voltage in my experience....even though the brains (so to speak) of the system may be a bit confused. The beauty of the Mustang TPS is the very low cost. But, as is so often the case, aftermarket versions may not be as reliable or long-lived as true OEM replacements. )
Anyhow, I disassembled the unit and found no visible physical damage or fault of any kind. I reinstalled it but the problem remained--- a slight jiggle of the wires caused the engine to stumble
I decided to go with the updated/later Jaguar TPS. "Genuine OEM", in Jaguar packaging. As it happens, I recently acquired a later style turntable (a different, unrelated, not yet told story!) so I was spared the expense of buying the adapter kit. I was hoping that this would be the end of the story, but it isn't. It can be adjusted to give the correct voltage at idle (.32-.36v) or the correct voltage at wide open throttle (4.5v or higher). It can't be adjusted to give both. As it stands I've have it set for .35v at idle and 4.2v at WOT, the best compromise I can manage.
I will say the the engine have never responded better to the throttle so I am very pleased as far as that goes. And.....the bump-bump-bump misfire sound in the exhaust is gone. I'm not sure what type of failure mode a TPS would suffer that would cause an idle misfire. A leaning-out condition, I reckon? That's a different discussion.
I will also say that the Jaguar-packaged part has no markings or identification of any kind. It looks exactly like the aftermarket replacements being sold for $65 less. Picture attached below. I am 98% sure it is the very same part being that is being sold in Brand-X packing at a lower cost. Label engineering. How long will it last? I don't know. I have to revisit the matter with respect to the WOT voltage. The problem here might be in the turntable or perhaps just some more adjusting after a good night's sleep. I'm not quite ready to condemn this "OEM" part but, if it can't be adjusted properly and/or doesn't last many years, heck, why did I spend $165.00? I might as well go back to the Mustang TPS for $30
Repeated engine stall when driving
The engine would die while driving, usually (not always) on closed throttle. This condition really disrupts driving pleasure
Again my assumption is that I had inadvertently bollixed-up some wiring or hoses in the process of replacing the vacuum advance. And, in fact, I did find some slightly iffy wires and connections, dutifully repaired or replaced..... and having no effect whatsoever on the stalling problem.
Since the problem occurred mostly on closed throttle my attention turned to the throttle bodies. The throttle blade clearance was OK at .002" but the bores were quite gunky. Ahhh-HA! I was sure that cleaning the bores would solve the problem....but it didn't.
Cutting to the chase, it was the ignition module. This was an aftermarket (NAPA/Echlin) module that I had installed a few months ago to replace a failed AC Delco module. Although the AC Delco is near-universally preferred I went with the Echlin module as it was immediately available at the time. None of the suppliers in my area keep the AC Delco part on-the-shelf these days; you have to order it and wait a couple days.
So....I ordered an AC Delco replacement, waited a couple days, installed it, and the stalling problem is 100% solved. But....perhaps that's not the end of the story.
Why did the AC Delco module I installed a few years ago fail? And how long will my new one last?
This module has been revised several times in the last 46 years. One visible change is that the unique grounding tab is no longer used. I've attached a picture below which shows the tab. The significance of this tab has been debated on many discussion forums. The AC Delco part that failed (prompting the ill-fated purchase of the Echlin part) did not have the tab. The AC Delco part I just installed does not have the tab. In fact, the part I just installed does not have any Delco or GM markings of any kind and has a generally different appearance than any Delco replacements I've used. Are we now getting a Brand-X part in AC Delco packaging? I dunno; I'm suspicious.
The "1875990" is the earliest iteration of the module, going back to the 70s. There are several later GM part numbers although the AC Delco "D1906" has remained the same through the decades. I don't know specifically when the ground tab was eliminated but I think it was within the last few years.
I have found and purchased a NOS 1875990 module with the tab. I'm not sure if I'll need it. I might install it and save my just-purchased version as a spare.
Unexplained high idle
Not resolved. Idling at 1000 rpm in "P". I dunno why. The throttles and linkages have been double checked. All vacuum hoses attached...or at least I think they are. If something has been left disconnected, it isn't obvious.
The car is actually driving very well now. I am going to drive it for a few days and actually get some enjoyment out of it and then come back to revisit the TPS adjustment and high idle issues. Often a fresh approach and fresh eyes will reveal something that was missed before. We'll see.
Doug
Is the high idle always there, or only sometimes? Congrats on the rest of it though!
Always higher than it was prior to replacing the vacuum advance, but the 1000 RPM in "P" is only after fully warmed up. And apparently not related to the vac advance specifically, as unplugging it makes no difference (I have the advance plumbed to ported vacuum, not direct vacuum)
The obvious first thought (for me, at least) is that I have extra air coming in from.....somewhere.
I've eliminated the possible of mechanical interference, throttles hanging up, that sort of thing.
I don't have an AAV or EAV.
Still there other possible sources I haven't looked at...yet.
Sorry to interrupt this program but where the part is made has no ill effect. China, Mexico, Argentina, South Korea, Brasil, Taiwan, you name it.
Ford and GM have plants in all those countries with trained assemblers who work for less money. More profit, less cost for the corporate boys.
those assemblers assemble and finish whatever is given to them to assemble and finish. They don't DESIGN the parts.
You can buy a brand new Ford and if you remove the brake rotors, they have MADE IN CHINA stamped on them.
Go to the Ford Dealer and buy a new Motorcraft Rotor and it has Made in China stamped on it, but inside Motorcraft packaging. Same with GM.
A good example is Fender Musical Instruments of California. An American Standard of electric guitars since 1948.
They started manufacturing Fender guitars in Japan and Mexico. What has happened is that the Made In Mexico and Japan Fender guitars have gained a reputation for excellence and now guitarists don't want the Made in USA Fender guitars due to poor quality. Go figure that.
Sounds more like Pandora's Box reading your trials and tribulations !
I blame myself for being too boastful about how well the car has been running and how it never gives me any trouble. You know the old saying. "Be humble, or be be humbled."
Run a socket over the Inlet Manifold nuts. That gasket can and does, settle, and a vac leak there is near impossible to zero in on. Has worked for me a few times.
I have that TPS you now have, ex an XJ40 3.6ltr. I am using the 0,34v at idle and forgot the rest. The engine is sweet, and the fuel cut off on high rev down hill overrun is quite noticeable.
I did not use that adaptor, STUPID $, so made my own.
The thought struck me, that the fast idle is not the result of a vacum leak, but advanced timing. Is that possible via the TPS!!! A new part added and adjustment needed...
Good thinking Carl but there's no tie-in between the TPS and the ignition timing....which I haven't tampered with (or even checked) since the beginning of this adventure.
Have not had time to look at the high idle problem. (1000 rpm in "P")
A new (sort of) problem developed. And it's deja vu all over again
Have been enjoying the car. Running great except for the high idle. Until, that is, yesterday afternoon. This wildly oscillating/surging running problem developed. When I opened the throttle just above idle the RPM would go up-down crazily. I have attached two videos.
The first video the symptom. Note that this is with the new "OEM" red TPS. Crazy RPM swings.
The second video shows the same scenario with the Mustang TPS reinstalled. No crazy RPM swings.
Now, just for fun...and a little stroll down memory lane.... take a look at this thread from Jan 2018. Posts 10 and 11. Look at the video in post #11 !
I have found and purchased a NOS 1875990 module with the tab. I'm not sure if I'll need it. I might install it and save my just-purchased version as a spare.
This order arrived today. Rather than the sending the item shown in the listing the seller tossed a unknown-origin module in an old Delco box. All markings have been crudely removed from the part.
Doug,
In the first video, towards the end the revs stop oscillating. Is this because you have you foot on the throttle at that point?
Next idea: The speed of the oscillation is quite fast, faster than if the overrun cutoff was the cause, it seems to me; I do not rule out some aspect of that though. What puzzles me is the high idle on the other TPS. So back to basics:
The only cause for a higher idle than required is more air that needed. It just MUST be getting in somehow. So in your situation, maybe with the Mustange TPS reinstalled, go round and block off every single spigot etc etc, not forgetting those under the TBs, the AAV feed at the AAV inlet, AND the balance pipe ECU inlet and put a MityVac on the ECU itself at the ECU. Even, if needed removing the balance pipe and blocking the 1 inch inlets to the manifold. At some point the idle will drop.
I do admire your sang-froid over this! If I had it I would be driven mad until I got it sorted out...
You said that you dont have an AAV etc, as I do not also.
Idle speed is set on all mine using the throttle disc stops and "balancing" the 2 as you would carbies. I dont run a balance pipe either.
That surging is weird at best, and as Greg said quite fast. The fuel cut off from the ECU is generally slower than that, but I really have never timed the surge.
Over run valves, front of each inlet, are a PITA, and if you still have them, I would delete them. Jag deleted, reintroduced, delete, these damn things over many years. All mine sharpened up once they were blanked off with simple alloy plates.
The vac supply to the ECU must be checked for holding Vac, but I doubt it ios at play here, as IF there was a leak there, teh ECU would sense that as load and supply more fuel.
PCV valve, and its associated plumbing.
Coffee #4 already:
Remember, the AAV when wide open as in cold, raises the idle by approx 400-450 rpm, and that thing has a BIG opening when cold. You are getting 1000 from a set of 750ish, so a vac opening for 250rpm would be BIG, or a LOT of little ones. Any spigot on the inlet rear and throttle body would maybe contribute 50-70rpm. 200ish would be 3/8 or bigger.
TPS issues you are having are ODD at best. My red thing in that write up came to me in 1997ish from an '88 XJ40 in a wrecker of unknown mileage, and is still OK. Prior to taht the car ran sweet with a Mazda 626 of ??year TPS that I adapted from a wreck also, as I refused to pay Jag $$ for a TPS. That Mazda one was in there for 2-3 years, no issues at all.
The Ford TPS units we got here were useless, as the lowest I could ever get one was 0.68v, and I tried heaps of them from various models, thanks to a mate at a dealer.
Doug,
In the first video, towards the end the revs stop oscillating. Is this because you have you foot on the throttle at that point?
No. It's because my foot is off the throttle.
Next idea: The speed of the oscillation is quite fast, faster than if the overrun cutoff was the cause, it seems to me; I do not rule out some aspect of that though. What puzzles me is the high idle on the other TPS. So back to basics:
The only cause for a higher idle than required is more air that needed. It just MUST be getting in somehow. So in your situation, maybe with the Mustange TPS reinstalled, go round and block off every single spigot etc etc, not forgetting those under the TBs, the AAV feed at the AAV inlet, AND the balance pipe ECU inlet and put a MityVac on the ECU itself at the ECU. Even, if needed removing the balance pipe and blocking the 1 inch inlets to the manifold. At some point the idle will drop.
I agree. That's what I'm planning to do, soon.
I do admire your sang-froid over this! If I had it I would be driven mad until I got it sorted out...
Well, the high idle problem is more of an annoyance than anything else. The other issues are/were more serious and intolerable. I'm pretty sure I can find the problem if/when I can dedicate some time to it. I've been picking away a few minutes here and a few minutes there.
Over run valves, front of each inlet, are a PITA, and if you still have them, I would delete them. Jag deleted, reintroduced, delete, these damn things over many years. All mine sharpened up once they were blanked off with simple alloy plates.
I've been wondering what would happen if the over-run springs became weak with age. I've been planning to delete them for some time. I'll move it up on my list.
The vac supply to the ECU must be checked for holding Vac, but I doubt it ios at play here, as IF there was a leak there, teh ECU would sense that as load and supply more fuel.
PCV valve, and its associated plumbing.
Coffee #4 already:
Remember, the AAV when wide open as in cold, raises the idle by approx 400-450 rpm, and that thing has a BIG opening when cold. You are getting 1000 from a set of 750ish, so a vac opening for 250rpm would be BIG, or a LOT of little ones. Any spigot on the inlet rear and throttle body would maybe contribute 50-70rpm. 200ish would be 3/8 or bigger.
You'd think a single large leak would create quite a hiss....which I'm not hearing. But, trust me, I'll leave no stone unturned !
TPS issues you are having are ODD at best. My red thing in that write up came to me in 1997ish from an '88 XJ40 in a wrecker of unknown mileage, and is still OK. Prior to taht the car ran sweet with a Mazda 626 of ??year TPS that I adapted from a wreck also, as I refused to pay Jag $$ for a TPS. That Mazda one was in there for 2-3 years, no issues at all.
The Ford TPS units we got here were useless, as the lowest I could ever get one was 0.68v, and I tried heaps of them from various models, thanks to a mate at a dealer.
Yes, odd. An the iffy quality of replacement parts these days adds another variable into the problem. I'm considering buying a used (but authentic Jaguar) TPS just to see what happens
I will run with this Mustang TPS for a bit until I figure out the high idle. I'll then revisit the matter.
The engine doesn't outwardly object to the higher-than-spec voltage at idle so it's tempting to just forget about it. Tempting, but impossible. It'll be an itch that I must eventually scratch.
03-20-2021, 10:50 AM
TPS and other woeful tales, trials, and tribulations
