I've had a California stock 76 2.0 for 30 years that runs perfectly. Every 2 years I have to smog it and it has always easily passed. I thought I heard that smog pumps were 'unobtainium', so I have been running it without the belt on for years, concerned about wearing out the air pump bearing or some other issue with it. BTW, I can perceive no difference in operation with or without the belt on driving the pump.
I used to smog it with the belt on, but last few times it passed the same without the belt on (the guy never checked for it...)
My question is: Is it wise to not operate the air pump and risk failure at some point and difficulty replacing it, or is there some aspect to keeping it spinning that I'm not considering?
ty
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Isn’t the pump just injecting fresh air into the exhaust port?
Seems like an odd way to reduce emissions….because it’s not actually doing that.

As far as not running anything for years….I would not advise it.
Maybe fire it up once a year for a bit to keep it functional.

Thats how majority of people in California run a smog pump bolted on and doing nothing so they can make it past inspection.

It pumps fresh air into the exhaust to burn up the rest of the unburnt fuel.
So if your car is running good in the first place you wont have much unburnt fuel anyways and, as you said, you car passes the same with/without it.

I dont know how difficult it is to remove and install these. But if you're worried about it remove it and only install during smog time.

Curious,
How does injecting air into the exhaust enable anything to burn?
Seems like it’s thinning out the exhaust gasses, but I’m not learned enough to understand how more air could enable burning fuel.

If the car is running rich by any amount, not perfectly stoich or lean, there will be unburnt fuel in the exhaust gases since all the available oxygen was spent during the combustion cycle.

The left over fuel in the exhaust gas is just sitting there waiting to burn. It's vaporized its above ignition temp and when the smog pump pushes in some fresh air the reaction continues and the rest of the fuel is spent.

So no disgusting gas smell coming out of your exhaust pipe. And with the addition of catalytic converters it can preform its reactions on all the exhaust and not have fuel coming through and damaging it

So that is why the 75/76 heat exchangers were super hot….by design to burn off gas vapors

A little history on secondary air injection systems aka smog pumps/pulse air systems. https://en.wikipedia.org/wiki/Secondary_air_injection

I guess you are changing what comes out of the tailpipe….but are you really reducing the bad gasses by burning them??

Seems that making the engine more efficient would be better overall.

But is it true that these smog pumps are hard to replace? Snd does running it increase the risk of eventual failure?

IE: Thermal Reactors ~

yes. you are.
but - its not really gases you are burning.
its unburnt hydrocarbons. tiny droplets or particles of long chain hydrocarbons.
ie fuel.

the gases get dealt with differently.
NOX - primary cause of smog was dealt with by ignition timing.
CO - carbon monoxide - by turning it into CO2 (slightly more harmless) and H20 using the cat. some hydrocarbons are also dealt with in the CAT by chemical conversion and absorption.

the 75 L jet did not need the air pump.
it ran EGR + CAT in california and did not have EGR or CAT in 49 states.
basically L jet could run more cleanly ( i don't think efficient is necessarily the right word but i guess sort of more efficient).

the D jet 2.0 california ran the smog pump, CAT and EGR.
the D jet 2.0 in 49 states ran only the smog pump i believe.

i guess D jet needed the smog pump at that time to make it through.

these were not sophisticated computers running the EFI.
though they were in their time and a lot more sophisticated than a carb.
but good as they were they still had trouble meeting EPA and CARB.

things got tougher in 76.
the 912E with the same 2.0L engine and running L jet needed a smog pump.
so i guess L jet couldn't quite get there by a year later either.

with smog regs the EPA and CARB revised their approach around 72 to cooperate with car manufacturers and available tech. they went after NOX first and delayed the stricter HC and CO limits. NOX was mainly done with ignition timing. thats what the double can distributor is all about.

they toughened up HC and CO emissions from 75 on in california (CARB) and 76 (49 states). primarily that is what the CAT was for. CARB records indicate porsche and VW had a bit of trouble with the CATS getting them to last the required 50,000m.
this may have been to do with the size of the cat they could install and also its location further along the exhaust system after the primary muffler. not really sure.
but the air injection and semi thermal reactor heat exchangers probably assisted the CAT to do its job by knocking some of the HC emissions out before they got to the CAT.
@914werke is correct re the heat exchangers being thermal reactors.
they were not full thermal reactors but sort of psuedo or quasi thermal reactors.
the 912E and the 911s at that time in 76 did get full thermal reactors which are can-like cylinder shaped things that hung under the engine. because they had full thermal reactors they did not have CATS fitted. thermal reactors were an alternative technology to CATS. mercedes benz fitted thermal reactors as did BMW.

thankfully australia escaped the thermal reactor pollution gear.
it was not good for engines i believe. made them run very hot.

Bingo.. they were found to be EXTREMELY detrimental to heads, cooking them (the heat had to go somewhere) & were said, in the case of the 911 to be a significant factor for the now infamous head stud issues.

Can anyone answer my original questions
1- Are these smog pumps hard to find in the marketplace?
2- If the pumps are hard to replace, does it make sense to not run the pump except for smog testing time?
Ty

it robs 2hp from the engine.
why the 75 is 88 hp not 90 like the 74.
so its an advantage not to run it.

i believe its a VW smog pump similar to what was fitted to buses, also beetles i think.
not something i go into being in australia. but i would have thought maybe no worse to try and find than the rest of our EFI parts. which in some cases is getting difficult.

you need the part number which is in the PET catalogue.
and then you need to sniff around.
places like the Samba might have sellers or ebay maybe.
check part numbers to see if a match.

-a further thought.
my bet is they are rebuildable.
most pumps are.
worst came to worst and you had to have the pump and it had to be operational for smog i would have thought it can be redeemed that way.
there are all sorts of pump rebuilders down here in aus.
for instance hydraulic pump reconditioners for citroens.
you don't tend to buy new pumps but rather take them in for reconditioning.
maybe a bit of sniffing around to see if your pump can still be reconditioned by someone in the USA? just a thought.

1) yes
2) yes

Thank you towle, that’s what I thought. I love simple answers to simple questions too, lol…

Not simple but the real answer. Not sure what it does to your car by the hot gas and that side of this question!

The smog pumps themselves are easy to get rebuilt (ebay) but the sacrificial rubber piece that connects from the pump by belt to the crankshaft is very difficult to get and are expensive . They are dying and nobody is making new ones.

if the pump freezes that piece of rubber busts up and the car continues to run. Disconnecting the V belt from the pump makes it so sacrificial piece of rubber will not be an issue.

Come smog time, connect the V belt and hope you pass. I had a pump freeze and kill the sacrificial rubber piece on my 76 car that eventually went to Europe for sale. I did get a replacement used and Lehman put it on my car. Not easy with the Engen in the car I tried it myself but I just couldn't get it.

Bob B

I would keep everything in place, disconnect the belt and spin the pump every so often by hand (like Richard mentioned previously)

Sorry, that is not correct.

The 74 made 95 HP (DIN spec), while the 75 made 88 HP (DIN). That's a difference of 7 HP, and the presence or absence of the smog pump is not specified. The 75-76 exhaust is horrible, so I am pretty certain that the power difference is due to that, not to the working of the pump.

As mentioned, spin the pump by hand every month or so. They can corrode (they're attached to a 914 after all!) and spinning it will help keep everything lubed and moving. Be very careful with the drive pulley for the pump, as those tend to shear and have been NLA for a very long time. You may be able to improvise something to repair one, but better to not need the repair in the first place.

--DD

I'm not that familiar with the air pump system used by Porsche, but on 1970's American cars there was always an anti-back flow ("gulper") one way valve used in the plumbing so the exhaust could not travel back into the air pump when the pump was not in operation. Simply disconnecting the fan belt from the air pump soon led to that anti-back flow valve failing from the constant flow of hot exhaust gas into it, which then had the cascading effect of allowing the hot exhaust to enter the air pump and cook its internals and bearings.

After checking the factory parts diagram I found that there is an anti-back flow valve used. It is Porsche Part #058-131-603, and the factory calls it a "check valve." It is attached to the metal piping from the air pump to the exhaust ports in the cylinder heads. Simply removing the fan belt will likely cause the eventual failure of that check valve, which will then allow the unobstructed flow of hot exhaust gases into the air pump and the air pump's subsequent death.

i can believe that.

use it or lose it.

looks like we are both wrong dave.
i certainly am. naughty me rounding down the USA 2.0 one hp.
thats the figure i use. we do metric here.

from my 74 owners manual.
admittedly these numbers are SAE.
i like to say my 1.8 has 76 hp. sounds better. but the book says 73.
so i'm guilty of bending the truth to save embarassment talking about my car.

smog pump is certainly a greedy little bugger if its sucking down that much hp but i can believe it. i was being charitable for once.
a good reason to piss it off if you could.
feel sorry for the californians!

@Dave_Darling

yeah, i just had a quick peek at a 75 sales brochure on jeff bowlsby's site.
that 75 2.0 is only 84 hp SAE.

7 horsepower gone missing!!!!
is that pump the whole villain or did they curb the d jet some other way.
thats a pretty big drag on the engine if it is the bad boy.
surprising because the 1.8s did not cop a power loss despite the EGR and the rest of it.

yep. combine thermal reactors with retard ignition at idle and cruise (which is what they were doing as well from 73/74 on) and its not nice.

my 74 1.8 runs noticeably hot at idle thanks to the emissions retard distributor. i try not to sit around idling in traffic too much when the weather is stinking hot down here.

in a perverse way the retarded ignition to knock back the N0X was causing the increase in HC at those idle (and in some versions, like the californian cars particularly, at cruise) - so they had to chase their own tails with the CAT and the reactor option to gobble up the unburnt fuel. i think even a by product of NOX reduction was an increase in CO.
its the reason the EPA stalled the reduction levels for HC and CO at a big pow wow they had with the manufacturers in 72. they realised what the mess was and decided to hold off a bit until the CATS could be brought in.

it wasn't called the malaise years for nothing.
VW did better than most i believe, but it still left a legacy that impacted on the air cooled engines. you can see why they didn't hesitate when bosch came knocking on their door back in the mid 60s with D jet. they would have been in real trouble without EFI.

I'm pretty sure that the exhaust is pretty much the whole problem. The late one is terrible, with no thought given to tuning at all--the exhaust ports dump into the pipe under the head unevenly, and go toward the front of the car, only to make a very tight 180-degree turn and head to the back. In addition, the left-side pipe crosses all the way over to the right before merging in.

This pretty much violates every rule that exists for making efficient exhausts...

--DD

@Dave_Darling

maybe.
except the 75 1.8 had the same shite heat exchanger system and it didn't lose 7 hp?
its quoted the same as the 74 1.8 officially speaking.

the poor old 2.0 really got beat up in 75.
if that pump was stealing the hp its not something i'd be leaving on there except as a hollywood movie prop to convince a smog station kook he can tick a box.



there must have been something else going down besides the awful exhaust system to knacker it that much? its a hell of a whack. 7 h.p.

Don't know why the 1.8 was not down-rated for 75. They could have stopped bothering with it when they decided the 1.8 was not going to continue past 75, or they just didn't want to admit how much they had neutered it?

Regardless, just look at the exhaust. It's horrible, violating most of the rules-of-thumb that I know of for exhaust tuning.

I don't believe the non-CA 75-76 914s had smog pumps, and they still had the lower power rating. Pretty sure it wasn't the pump that did it.

--DD

the exhaust is horrible for sure.
you won't get any argument from me about that.
horrible and expensive.

i can see where you are coming from.
the 73 and 74 2.0 did have a better muffler than the standard 1.7 and 1.8.
more like a 911 muffler. less restrictive. the 1.7s and 1.8s might have had nice looking extractor heat exchangers but they fed into a pretty restrictive standard VW muffler same as the buses and the type 4 sedans. so maybe you are right. less impact on the 1.8s as they were no great shakes anyway in previous years but a much bigger impact on the 2.0s

1975 service and training manual indicates all 2.0s (USA and california) had the air pump.

the difference for california was EGR and a CAT.
those 2 items are all that is extra to californian cars (1.8 and 2.0)
the only added restriction in the exhaust was the CAT in california and given power is quoted the same can't have been significant..

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must be a shared impact between the smog pump and the restrictive exhaust system that resulted in the power loss for the 2.0s USA wide. probably mostly in the mufflers rather than the heat exchanger section, despite the crappiness of the exchangers.

Interesting, I had been under the impression that the smog pump was a CA-only deal!

--DD

yes, @JeffBowlsby has some fab documents on his website which are useful for establishing what was going on.

we used some of them to get to the bottom of the mystery of the 74 1.8s and the two very slightly different engines for 74 which had previously been glossed over in discussions over the years.

generally speaking the 49 states cars adopt the design and emissions equipment of the californian cars of the year before. this more or less happens to the 75 1.8s. they are set up identical to the 74 californians. with some differences. the decel valve gets redesigned and they cop the awful exhaust system. but it seems to make no difference to their maximum hp. however there may have been a difference in driveability and response. i can't say. never driven a 75 car. i have driven earlier 914s but nothing beyond a 74 model.

don't the air injectors shoot air into the exhaust port of the 2.0 liter heads?
I can't imagine that helps with a smooth exhaust airflow, and could contribute to lower HP.

Just considering how much HP can be had by porting and polishing a set of heads....versus introducing a thing sticking into that port/airflow.

Rich