Rhythmic Surge

[SouthwestSpider]

I just replaced the head gasket on my 80 Spider (FI). I've got it all back together. After taking it for a good run, when I parked I noticed the engine surges in a rhythmic pattern fast/slow as the car sits at idle. This has happened twice so far. The engine is hot but stable at 190. The timing has been set correctly, there are no holes or leaks in the air intake, although I'm certain that I could dial down the idle a little more - but the surging pattern has me baffled. It's not loud - just consistent and doesn't seem to be prevalent when the motor is first started and cold. Any ideas?

[rlux4]

It could be your AFM flap is bouncing. Take off the boot at the AFM and move the flap by hand. It should move smoothly, the slightest catch will affect it's movement and consequetly the signal the ECU receives. If it's not moving smoothly you should clean any gunk in the plenum and lube the pivot with a non greasy lubricant (silicone or spray graphite, not WD40).
Ron

[So Cal Mark]

set the idle speed to spec and make sure the tps is adjusted properly

[SouthwestSpider]

thanks I will check the AFM. I apologize for not knowing this Mark - but what is 'tps' ?

[RRoller123]

throttle position/plate switch

[rlux4]

You need to have a manual or two to help you maintain your Spider. Meanwhile there's a good FI section in Brad Artigue's "Fiat Maintenance and Modification" manual. You can download the FI section and it'll give you the method for setting your idle and adjusting the TPS.
http://www.artigue.com/fiatcontent/book ... 124_MM.pdf
Ron

[SouthwestSpider]

Excellent - thanks.

[bradartigue]

Do the things above.

Also check that the o-ring on the idle speed set screw is good and that the screw and throttle stop are set to spec. These things out of whack and/or an air leak will always cause a rhythmic surge.

[narfire]

Question.. when setting the tps, the throttle screw should be backed right off. correct.? I tried setting mine yesterday and got the resistance down but did not go all the way to 0 and the sensor was at the stops. I still have a miss or slight lurch/surge when idling along in second.
Sorry about the hi-jack but my issue might be similar.
Chris

[SouthwestSpider]

Sounds like a good thing to ask narfire.
Brad I managed to get the idle down to 1000 rpm but either my tach is goofy or ? I can't get the idle below 1000. The TPS (see Mark I learn good) is set. When I undo the throttle stop (with the idle speed set screw all the way in) and the throttle cable slack - I dont' get the idle any slower. I have the timing set correctly. It runs best at around 1200 rpm. Is it possible the tach doesn't read correctly?
It seems like it would be an air leak - it's as though when the afm opens the motor falters, very similar to the way it would behave if you undid the intake hose at the afm and pulled it off slightly.

[So Cal Mark]

the tachs are not accurate, and the throttle stop screw should not be used to set idle speed.

[narfire]

What I did... Used timing light with rpm/advance capability.
screwed the air mixture screw in as far as it goes but not seated hard.
backed the throttle stop screw off the throttle plate.
cable was loose.
Set the tps as per instructions ie spade conection #18 and #2, rotate clockwise till Ohm reads 0 or close to it, screw tight. This is where mine only went to .6 or .5. Going to have to try again tomorrow I guess.
Using the timing light, backed the air screw out until I got around 900 rpm.(engine was warm)
snugged the cable up so just a slight bit of slack and tightened the stop bolt.
Didn't like the idle, felt like it was going to stall. I used to have it around 1200 and was comfortable at the traffic lights.
Driving home still felt the lurches and surges when driving slow down Lakeshore while looking at the scenery
I'll do a leak test tomorrow at the shop to see if the plastic has a crack,loose conections ect. I'll check the clearances as well on the cams. A fellow at Can. Tire commented/asked why the rough idle.
I have another tps, perhaps try that?

[bradartigue]

If you cannot get below 1000 RPM then your ignition advance is likely too far advanced (it should be at 10 BTDC) or you have an air leak or your throttle stop screw and idle speed adjustment screw are messed up. While agree with the tachometer comment Mark makes below the second comment about the idle speed contradicts the shop manual (and my book, but my procedure is exactly the same):

the tachs are not accurate, and the throttle stop screw should not be used to set idle speed.

The throttle stop screw sets the position of the throttle at rest, approximately 1 degree of opening or the equivalent of 800 RPM. The idle speed adjustment screw is then used to bring up the last 50 RPM.

As so:

Run the engine to normal operating temperature. On cars with automatic transmissions, set the parking brake, block the wheels, and put the transmission in "D". Completely seat the idle speed screw. Loosen the accelerator linkage stop screw retaining nut and adjust the stop screw to achieve 800 to 900 RPM on manual transmissions and 700 to 800 RPM on automatic transmissions.

Lock the accelerator stop screw with the retaining nut. Use the idle speed screw (1) to set idle at 850 to 900 RPM on manual transmissions and 700 to 800 RPM on automatic transmissions. If idle speed cannot be reached using this procedure, fully remove the idle speed screw (2) and clean the orifice and the screw itself using a high pressure spray cleaner (carburetor cleaner is fine).

Here is a much longer process I described in http://www.fiatspider.com/f08/viewtopic.php?f=5&t=18519 - "Step Zero" part 1 about the brake booster may not apply to you, but it is the right place to start.

Step Zero: Test your brake booster connection. Unplug the brake booster hose at the manifold and block it off with a cap. Drive the car. Did the problem go away? Keep in mind the brakes will be a little harder to press but if the car runs perfectly or very differently (high/low idle, etc) with it unhooked then it leaks; fix it.

If that isn't it then remove the air bypass screw and clean the orifice and the screw. Replace the o-ring. Fully seat the idle air bypass screw. Slacken the throttle cable nuts so there is no tension on the throttle. Start the car and warm to normal (two fan cycles). Set the stop screw to 800 RPM. Set the air bypass screw to bring the idle to 850 RPM.

Drive it. Is the problem resolved? Yes, great, no, read on:

Test the throttle position switch (I can't recall the values, go to http://www.artigue.com/fiat and download the book I wrote on it, or look in your shop manual). If off then set correctly.

Drive it. Is the problem resolved? Yes, great, no, read on:

Stop the car and manually advance the motor to line up the crank at TDC and the cams at the alignment holes. Does it line up perfectly? No? Reset timing belt. Yes? Attach timing light and start motor. Set timing to 10 degrees behind top dead center with the diaphragm vacuum line pinched.

Drive it. Is the problem resolved? Yes, great, no, read on:

Stop the motor. Check that the distributor vacuum advance works by drawing vacuum. Does it hold? If you don't have a handpump then blow into the line (don't suck - if the diaphragm is bad you'll suck in dirt!). Solid? If not then replace. If so then read the plugs. Brown to brownish white = normal; black = rich; white = lean. Which is it?

Brown to brownish white? Yes, great, no, read on:

Test the AAR operation (see my book). Using propane (not fire, just a light amount of propane from a small tank) around the air hoses try and find a leak. If the engine speed goes up when you place propane near the hose then you've found one.

[So Cal Mark]

perhaps I wasn't clear; the stop screw does have a setting but it shouldn't be used to set the idle screw. The idle air bypass is for setting the final idle speed. The stop screw prevents the throttle plate from sticking in the throttle body if it closes completely

[narfire]

I'm at the shop, (so much for the honey -do-list eh) . Have the throttle screw backed all the way out. Have the cable loose.
Trying to set the tps with no luck. Meter starts at one, cross the leads at goes to 0. Touch the leads on the tps and reads .5 to .6 at best. Move the tps clockwise and no difference on the meter. tps cannot go counter-clockwise any more to see if I can get a reading of 1.
I'll see what the other tps is like on the spare intake.