Rough & Hunting Idle problem on my 83

unclemikeys2001 · 05:52 PM

3 weeks ago I swapped an 84 n/a motor into my 83 from a wrecked donor car that still ran excellent. I had new belts, water pump & speed & ref sensors installed by the owner of a local Import shop while the motor was out. I installed the motor & all went well. Can ran great for the 1st 300 miles after swap then began progressively to become harder to start but will always start (warm or cold) & has developed a hunting surging idle going up & down from about 950rpm to 400rpm idle. Car now has about 500 miles since swap. This problem does not happen all the time but is like 7 out of 10 times. The idle sometimes smoothes out perfect once I drive a bit or if I hold the rpms up high for 10 seconds. When driving at any speed the car runs great with no idle problems & is very strong & has lots of power but as soon as I come to a stop the idle starts to go up & down again almost dying out sometimes. ....What Ive done so far: Replaced DME relay, cap, rotor, plugs, Swapped out AFM with a known good unit, inspected all vacuum & fuel lines, replaced fuel filter, checked/cleaned all grounds & connections. ...I called the guy who installed my speed/ref sensors & he assured me he set them correctly. He said that the tempeture sensor or 02 sensor are likely bad. However I doubt the temp sensor is bad & the 02 sensor has less than 3000 miles but who knows how long these last. .......Please let me know what you pros think? Im very new to 944s but am catching on quick due to the fact that Im working on it more than driving it. HELP ME. Thanks!!!!

Moneyguy1 · 09-10-2005, 06:02 PM

"Working on it more than driving it"....

Welcome to the gang!!

I would suspect a vacuum leak as a first start.

Do you have the parts from the "old" engine? You can swap out the "barn door" and throttle body one by one. Could be fuel related...How old is the filter? Have you checked fuel pressure?

Make of a list of "the usual suspects" and try them one at a time. That way you will be able to identify the culprit when you finally locate it.

ae1969 · 09-10-2005, 06:56 PM

........... when the idle starts to hunt like you describe it is normally caused by vacuum leaks after the throttle-body. This usually causes the air-fuel ratio to go out of whack and the DME tries to compensate with fuel............then it throws off the idle speed .......... and then the idle-stabilizer tries to compensate and that in turn throws off the air-fuel ratio again... etc...

Now you have a lot of places too look for leaks........
- vacuum hoses
- idle stabilizer (the mechanism itself)
- intake-manifold gaskets
- the big hoses under the intake manifold, brake booster etc

.............. and sometimes a faulty TPS can cause similar symptoms

SoCal Driver · SoCal Driver's Garage

83-84 does not have an active idle stabilizer valve. It's a simple heated spring single action valve. 85.5 and later for the active valve.

Check the adjustment on the throttle possiton switch. should click when the throttle seats. Also check the large screw on the throttle body. This is a bypass to set the basic idle. The o-ring may be bad and the screw has come loose. Might just be barely sitting in it's bore.

Did the AFS come out of an early 944?

unclemikeys2001 · 09-11-2005, 10:31 AM

Yes, the AFM is from an early 944. .......How do I know what to set the basic idle at? Im just afraid to start messing with it before I know where to set it to exactly. ...I drove it about 30 miles last night & it was running great the whole time except for a brief moment. If this were a vacuum problem wouldnt the idle be messed up at all times, & not just when it wants to? Thanks in advance for the help. Im going to work on the car today & will post what I find.

unclemikeys2001 · 09-11-2005, 12:42 PM

After running great last night Im back to the car running like crap today. I just checked the throttle position & it clicks when seated, seems fine. I also checked the basic idle screw & the o-ring is good. I pulled it all the way out & it was full of carbon. I cleaned & reset this & still the same situation with surging idle. ...I also inspected the entire engine bay for vacuum leaks & can come up with nothing. I dont believe its a vacuum leak. ...I pulled the spark plugs just to take a look at them & my plugs with aprox 500 miles on them were very badly fouled with carbon. I cleaned them & re-installed & car runs a little better but I know they will probally just quickly foul again. ....An interesting note that I forgot to mention before is that upon morning cold start up you can smell unburnt gas from the exhaust for the 1st few seconds. Surely due to the fact that I have to crank the engine longer than normal to get it to start. Sometimes it helps to give a little gas while cranking then it starts. ....Do you guys think this problem has anything to do with the CO setting? Im not so sure because I have 2 good AFMs & swapping them doesnt make the car run any different. I dont want to mess with the CO setting because I dont have the tools needed to corectly set. This is very frustrating... Why do 944s have to be so fussy!!!! I appreciate all the imput, so please keep it coming.

SoCal Driver · SoCal Driver's Garage

Unburnt fuel smell is a good indicator of a bad fuel pressure regulator or the vacuum to the FPR. These can be intermintent in their Bad Boy behavior.

ae1969 · 09-11-2005, 07:21 PM

Unplug the O2 sensor........

onZedge · 09-11-2005, 09:51 PM

I had a similar problem. After replacing al the vacuum HOSES, it ran much much better except for a slight hunting at idle. I still suspected a vacuum leak so I replaced the vacuum TUBING, which I thought LOOKED to be in good shape. Problem solved. I used 3/16" fuel line from Lowes (shiny black stuff) and black 75# ty-wraps as "hoze clamps" as well as a couple of black nylon tees from the aquarium department at Walmart. I ran it over the intake manifold and used 3/8" nylon p-clamps at the throttle cable mounting plate securing bolt and that threaded hole in the top of the intake manifold to secure (that is if you don't have cruise control). I'll send pictures later as I would make a racket doing so. My wife has to work early tomorrow and needs her sleep!

Slam · 09-12-2005, 07:52 AM

I'd also R&R the J-boot and the auxiliary air valve as well. Mine did exactly what you're describing and this fixed it.

unclemikeys2001 · 09-12-2005, 12:16 PM

I have just done several things that were suggested. Ive Unplugged the 02 sensor & this does absolutely nothing to the idle. It makes the car run no better or worse at any temps. Ive removed & replaced the J-boot & cleaned the throttle body. My throttle body had some gassy oil sitting in the bottom of it. I will find out in a few days if this returns. I also pinched close the Auxilary Air Valve line & this does absolutely nothing. According to the Haynes I have it says the idle should drop once pinched shut, my idle remains the same. ....If anyone knows about the 02 & Aux air valve situation let me know. ....Ive also located a vacuum line that is not connected to anything. I cannot locate the name of this part or where its supposed to go. Ive included a photo of where the line sources from, my finger is on the line. (if the pic uploads) The line then travels along the firewall & ends about where the speed & ref sensors plug in. The source is a small plastic thingy that is near the brake booster. Help me out if you know where this goes. ...I dont have a fuel pressure tester or else I would have gotten some readings by now, Im working on getting one. Thanks again for all the suggestions, please keep em coming.

rb101 · 09-12-2005, 03:23 PM

That is the vacuum valve for the fuel evaporative container thingy emission crap (technical jargon)

SoCal Driver · SoCal Driver's Garage

Looking at the picture me thinks it might be a bad brake booster diaphram.

onZedge · 09-12-2005, 08:44 PM

OK, that small plastic thingy valve is the evaporative emission shut-off control valve. The line your finger is on goes to a temperature controlled vacuum switch located in a small coolant line under and between intake runners 3 & 4. The vacuum switch has two nipples on it. One points up and the other one points up at an angle. That vacuum line your finger is pointing to goes to the nipple that points up. The nipple that points up at an angle connects to the throttle body nipple that points forward at an angle. (Angled nipple to angled nipple.)

The job of the small plastic thingy valve is to shut off vapor flow to the engine when (1) the engine is cold and (2) the engine is hot and at idle. The only time the small plastic thingy valve is open is when the engine is hot and your foot is on the gas.

If the line your finger is on is disconnected, the small plastic thingy valve should always be closed no matter what. I think the small plastic thingy valve is faulty which is allowing vapor flow to the engine all the time no matter what.

To troubleshoot this, first locate the small metal thingy that the small plastic thingy is connected to with a short big fat vacuum hoze. That's the evaporative emissions flow control valve in your picture bolted to the inner fender. Disconnect the OTHER big fat vacuum hoze from the small metal thingy that leads to the j-boot and plug it.

My reasoning is that if it idles better with the line plugged, it is because at idle, you were getting a vapor enriched air supply you weren't supposed to. This is probably why your throttle body also has oily residue in it. Remember though that if you leave the line plugged, you will get a strong fuel odor and even a fuel leak at the vacuum canister when it becomes fully saturated with condensed fuel vapor.

The job of the evaporative emissions control system is to prevent this.

Of course, when the fuel pressure regulator starts to fail, it fails "high" and the high fuel pressure would also cause it to run crappy at idle.

And finally, myriad other things can cause crappy idle. These are just a few.

"Now he's one of us!"

unclemikeys2001 · 09-13-2005, 08:18 AM

I think I have my problem fixed. I swapped out my Fuel Pressure Regulator from my parts motor & it runs much, much better. Still not 100% but it definately is running a hell of alot better than the past week. There is still a very minor idle miss but nobody else can really tell but me, its slight. Slight enough for me not to really care right now. I may have other smaller issues that I havent found yet. ...While I was at it I connected the loose vacuum hose as well. This doesnt appear to affect the running condition but Im sure it doesnt hurt to have it all hooked up correctly either. ...Well, for now all seems well. Thanks to all of those on this site who helped me out. Without you guys Id probally be wasting alot of money by taking the car to a shop.

bearone2 · 09-13-2005, 09:01 AM

not sure if you have the bottom protective cover on/off.

my 84 would surge/run rough/die at idle with it removed for some reason(not after driving in the rain) but had no problems with it on.

unclemikeys2001 · 09-13-2005, 09:13 AM

Its on & has been since the motor swap. I wouldnt dare leave it off too long, Id just hate to run something over & be replacing god knows what. Everything hangs to low for my comfort on the crappy roads around here.