Engine Revs Up + Down at Idle

[eR1c]

I have a '77 930 w/ 3.0 engine (it's a late model 77 w/ 78 engine).

Car backfired and unsnapped air box and mangled the air filter. This was because of a starting issue I had, I sprayed some gas down the intake, yet it backfired (in both senses of the word). I later found that the starting issue was an inexpensive fuel pump relay.

So, new fuel pump relay and the car starts no problem. Runs great, lot's of power, etc.

The issue I am having now is that only at idle the engine revs up and down at a constant pace from about 3000 rpm to about 1200 rpm. When driving there is no issue, the car cruises at a nice rpm around 2200 rpm when not giving it gas.

I checked the throttle assembly + spring at idle and it's not moving. My suspision is a vacuum leak? Or some kind of vacuum solenoid? I checked the vacuum line and it appears to be connected. I suspect the backfire caused something in the vacuum process to break or a hose to become disconnected or a solenoid to break??

I'd appreciate your guys help on this,
Thanks.

[myamoto1]

Check for air leaks around your intakes. There's a good chance your backfire broke a seal and you're getting un-metered air coming into the combustion chamber. The two methods I know of are:
1) Listen for a "sucking sound" around your intakes. This is safe, but sometimes hard to detect over the sound of the enginge.
2) Spray starter fluid around your intakes. If the revs go up, you've found your issue. However, this is not nearly as safe as option 1.

[eR1c]

I can hear a general sucking sound ...pinpointing it is going to be fun.

when I check for leaks around the intakes ...you mean all the intakes into the air box and manifold .... I'll re-tighten everything and keep looking/listening.

I don't like the idea of spraying starter fluid around ..that's how I got the backfire in the first place.

[Akira28]

The garden hose technique works pretty well (take a small length one end to ear and the other to listen to your suspected source). My wife is a nurse so i took one of her old stethiscopes and inserted a metal rod into the diaphram hole and it works like a charm. Harborfrieght also sells the same (cheepy) setup for 5 bucks in their automotive section.

If you smoke, light up, fill your mouth (leave the cig on the workbench ashtray and blow a big slow puff into your suspect area and see where is gets sucked in. Granted I'm not an advocate of allowing your car to smoke, but mines over 21 and prone to doing it anyway.

Akira28
_______
1983 911-SC Targa
1984 944-NA

[eR1c]

my wife is a nurse also, i'll see if I can liberate one of her stethiscopes

...taking the air box off (again) to see if everyting is tight ...sounds like the leak is coming from behind it somehwere.

[Mr9146]

Sensor plate

[eR1c]

I just noticed one thing that is amiss.

The intake manifold ...there is an air tube to the left of the large main tube. This tube looks to be snug, yet one end has the hose clamp missing. I can see that the hose clamp is partially still there ..the screw is there, yet the actual clamp is totally missing. I am wondering if the backfire blew it off ...as it was probably near coming off anyway as it's very old and brittle.

Once the engine cools completely I am going to tighten w/ a new hose clamp. I am hoping this is the issue? It's where about the sound was coming from in a general way.

Sensor plate, ...you mean some part inside under the plate? would that only effect idle though?

[longhornchris04]

I'm not sure exactly on a 930, but on the SC motors, its on the right-half of the airbox. I think its somewhere else on the turbos. Check the host's parts diagrams, I'm sure you can find it.

Assuming your CIS was set properly, its gotta be an air leak... possibly a cracked airbox if you had a bad enough backfire and don't have a relief valve.

Additionally, if your car is too rich, the idle will oscillate. Given the wild oscillations, this probably isn't your problem.

[eR1c]

i looked everything over, can't find anything apparent or visibly not connected.

The air box is tight and sealed. I've double checked it for leaks. there are none.

I am stumped. I may just bring it to a porsche mechanic and let them figure it out. It's getting a bit frustrating trying to sort this issue out.

[benzyne]

Could it be a rich or lean surge then if there isn't an air leak?

[eR1c]

that's what it seems like.

I am pretty sure there is no air leak. something happened when it backfired though. it does seem to be running rich/lean.

I am going to take it to "S-Car-Go" in San Rafael, CA (it's near where I live). They build race porsches and specialize in 930's + 911's. I think I could spend a lot of time and frustration figuring this out. -not like I already haven't. At this point I think It may be easiest to have them look at it.

[Mr9146]

I suspect a warped sensor plate as a result of the backfire. It will affect the mixture and may produce the situation you're dealing with.

I have seen this before...


I am under the assumption you have a 3.0 Turbo.

[eR1c]

warped sensor plate. ...it doesn't appear warped and appears to close completely flush. yet would it be noticeable if it were wapred ever so slightly? And if so, is it just a matter of replacing the plate? ...I think the plates are relatively cheap on our host's site. I assume they are easy to replace (looks like I just have to unscrew some small bolts) -is there anything else involved in replacing the plate ...like anything that needs to be tuned or any special tools?
(yes, it's a 3.0 turbo)

[Mr9146]

They have to fit pretty darn near perfect all the way around and sometimes it's hard to see.

Run the car without the air cleaner and if the sensor plate "flutters" dramatically it may be warped.

Also, you could take the plate out and lay it on a flat piece of glass or something and look for warpage as well, but I think it's just easier to run the car without the air cleaner first.

BTW, how does the car boost since the backfire?

[eR1c]

the boost seems to be working fine ..yet I only drove it for about 15 min's and only really got on it about 2x. The car seemed to have power. The odd idlying is only in 1st gear ...it idles pretty normally at all other gears.

Doesn't look like our host carries the part any longer. I assume that plate can be machined/straightened?

I am thinking that this sounds like a good possiblity though.

[Joe Bob]

Lean hunt....use a can of quick start...SPARINGLY....on areas that may have a leak. Change in idle...you found it.....

[eR1c]

I am done w/ Quickstart!
Quickstart is what caused the backfire and this idlying issue to begin with.

I think at this point since I can't find the sensor plate on any site including our host I am going to take it to a shop that specializes in performance porsches (they race them at Infineon Raceway ...they are really reputable and good).

I can't tell you guys how much I appreciate all the help. This site has already saved me several thousand dollars at least over the past couple years.

[Mr9146]

You won't find a new sensor plate unless someone has one rat-holed away somewhere. Trust me.

However, a good used one will do the trick just fine ... IF that turns out to be your problem.

Keep me posted (even if you have to PM me); I'm curious to see how this one shakes out.

[eR1c]

thanks I will keep you posted.

If it is a sensor plate and they can't replace it because they are hard to find, can't it be straightened or ground down a little on both sides to get it straight?

Anyway, i'll find out what it is exactly once I bring it in.

[Mr9146]

It's a waste of time to straighten it. Just get a good used one ... they're abundant. New ones? Rare.