83 FI Revving at Idle

[kcbodine]

I am having a issue with my 1983 Fiat spider FI revving at idle, The car starts up then after 5 seconds or so the engine just starts to rev to 2000 rpm and rev back down to 1000 rpm and so on without me touching the throttle. When put under load however the engine smooths right out and responds to the throttle exactly when driving, though starts to do the weird revving thing when stopping at lights or stop signs. The engine was just rebuilt with new seals all around, new injectors and new ignition parts, that leaves me to believe its something with fuel/air but I'm not sure.

Any info is great Thanks!

[wizard124]

There are some excellent resources on the web for trouble shooting Bosch fuel injection systems. Worth a search if your issues are not resolved easily.

A quick and dirty description is, as more air is drawn into the engine, more fuel is metered/injected into the combustion chamber. The throttle cable connects to the throttle plate. As the plate opens and more air is drawn in, this opens the flapper on the air flow sensor. The more the flapper opens, the more fuel is injected.

Do this without the engine running: The throttle cable is atop the intake manifold on the driver's side. Find this and follow it to the throttle plate. This can be rotated by hand but its spring should return it to its stop. My cable has a little slack to it and the plate returns to the stop/set screw. As you move it slightly, you should hear some contacts making a clicking noise. This is normal. Engine running: Watch this plate closely at idle. If it moves around on its own then I would fix this first.

The Air Flow Sensor is above your air filter. The flapper is also spring loaded to its idle position. If this wanders or does not seat properly, it could affect your idle performance. Without the engine running: Disconnect the top as if you are replacing the filter. Reach in and feel the flapper. It should move easily but return to its stop without binding.

Inspect the air plenum for loose clamps or cracks in it. Inspect the the attached hoses for cracks or leaks around their unions into the air plenum.

If all is good so far, I would do the following checks with the engine running, at normal operating temperature.
The next 3 things to check also can affect air/fuel. The hose to the power brake booster attaches to a fitting on the intake manifold. Take some pliers and pinch this shut. Does the idle smooth out and become stable?
Check the cold start valve to see if remains open. This valve adds fuel when the engine is cold. Locate the fuel rail. At the forward end is a short length of hose leading to the CSV that is attached at the forward end of the intake manifold. Pinch this closed and see if the idle steadies.
The Auxiliary Air Valve is also a cold start device and closes to restrict air after cold starts. It is attached by air hoses; one side goes to the main air plenum; the other to the intake manifold. Pinch off the intake manifold side for results.

If none of these steps isolate your problem.......it becomes more involved checking sensors with multimeters.

[rridge]

There's something called a Throttle Position Sensor (TPS) on the front side of the throttle body attached to the end of the throttle shaft. The name is misleading. There is no sensor. Instead the black plastic housing contains two micro-switches. One signals when full throttle is reached to trigger FI enrichment, the second switch signals that the throttle is fully closed. When that switch is triggered the FI system goes into its stable idle mode and takes the oxygen sensor out of the loop. The TPS can be mis-adjusted in its position on the shaft. The closed throttle switch can be defective. The wiring harness connector contacts can be dirty. Find a copy of the Fuel Injection Diagnostics Manual for full details.

[76was124]

Could be a number is things as mentioned, TPS, air leaks, vacuum leaks, distributor, AFM. Good post by RRoller copied below. All worth a read before tinkering, as the car runs, and changing too many things can cause a bigger problem.
Disregard the first part of the post, pick up at the paragraph on the TPS and of course the links to manuals

Interesting that the CSI fires but not the 4 injectors. I would guess a couple of things (GUESS I said.... ) I am guessing that the CSI fires based upon the TTS, explaining why it fires but not the 4 injectors, so it may be that the AFM is not sending the proper signal, maybe connector bad, internals, whatever.

You might measure the injector signals back at the ECU, that would let you know if it is a break in the wiring somewhere downstream. Also might isolate the problem to the ECU.

Also check the TPS at the throttle plate. It should be adjusted such that you can hear it click just as the throttle is opened. It tells the ECU to shut down the injectors when the throttle is closed. (To prevent backfiring, such as might happen when suddenly pulling off of the throttle while up at highway speed, for example). If it isn't right, the Injectors might be permanently closed?

Here are some Bosch L-Jetronics references online: (I personally think the best one is the last one)

FUEL INJECTION DEBUGGING
http://www.hiperformancestore.com/Ljetronic.htm
http://www.njfiats.org/joomla/images/st ... BoschL.pdf

FUEL INJECTOR CLEANING:
http://WWW.OKINJECTORS.COM
http://www.witchhunter.com

FUEL INJECTION MANUAL
http://www.mirafiori.com/faq/fiatFI_may2002.pdf

FUEL INJECTORS CHART
http://users.erols.com/srweiss/tableifc.htm

L-JETRONIC MANUAL
http://www.hiperformancestore.com/Ljetronic.htm

L-JETRONIC
http://mastertechmag.com/pdf/1988/11nov ... ronic2.pdf
http://jenniskens.livedsl.nl/Technical/ ... tronic.pdf

L-JETRONIC FI MANUAL (FROM VW)
https://www.type4.org/manuals/ljet/index.html

BOSCH L-JETRONIC MANUAL – BEST ONE:
http://foxed.ca/rx7manual/manuals/BOSCH ... Manual.pdf

http://dtec.net.au/Tech%20Articles.htm