Loping/poor idle on a stockport EFI 13b
 

I've noticed that over the last few months, my car has developed a 'brap-brap' kind of idle. This seems abnormal for a stockport engine. Any ideas what could be causing it? The car is a mostly stock GSL-SE

Check compression with a gauge, even a piston one and get back to us.

Don't think it's that. Engine runs very healthy above 2000rpm. Pulls hard all the way to 7500 and beyond.

Another thing that could be related is that sometimes the engine stumbles a bit below 1500-2000 rpm but instantly smooths out above that

Check the injectors, you can make any stockport rx "brap" by loading up fuel at idle. If you have an injector that is sticking it may very well be your culprit.


Isaac

That's interesting, because I feel like the car is running rich. Very poor fuel mileage (12mpg around town, at best) and very 'rich' smelling exhaust. How would one test that?

Does sound like over fuelling on idle, my 12A brapped when on choke. You need to get it on a wideband really I guess.

I would just swap in a known good set and call it a day, setting up a test area for injectors is a bit complicated if you aren't doing it often. Simply pulling them out and cleaning them may show you there is an obvious problem.

All of the symptoms you describe say sticking/leaking injector.

Isaac

Two things; injectors (as others have mentioned), and 'Surging' (search on that one).

First - the SE's have 2 injectors located under the Upper Intake Manifold (UIM) and if your car has anywhere near normal mileage, and it's on the original injectors, it's a good bet that one or both is bad, and certain bet that they're out of matching flow rate. One fine weekend, pull your UIM to get to the injectors and pull them after removing the fuel rail. Send them off to RC-Engineering or another place that tests/cleans/flow-balances them, and you'll see an immediate improvement in performance and fuel economy. The SE's can be reasonably expected to get about 18-19mpg city and as high as 24-26mpg highway, so if you're getting considerably below that with a stock setup and normal driving habits - look into it.

With gasoline at close to $4/gal, it's not hard to recoup the cost of new fuel injectors in the $$$ you'll save by replacing them now instead of waiting.

On 'Surging' - the SE's also have a cold-start mechanism that replaced the choke system on carb'd engines; this is comprised of several coolant based temperature systems, a mechanical 'fast-idle-cam' which is set by pressing the gas pedal to the floor ONCE, and then releasing it to start the car when cold, and by 'Secondary Throttles' which are actually choke plates in the air path which are opened by the Thermowax Pellet and fast-idle-cam release once the engine heats up. Short version of how to fix 'Surging in SE's' - you need to lube the steel throttle rods where they enter the aluminum throttle body. The rods stick when it's cold out, creating a feedback loop that causes the surging idle which usually runs up and down between 800rpm and 2000rpm or more, sometimes. It's caused by galling between dissimilar metals, micro-corrosion, and complex chemical reactions and mathematical equations that primitive minds can't comprehend...

When you lube the steel rods where they enter the throttle body - it magically fixes it (for a few years). Good luck,

Well I have no surging. The idle is low (500-600rpm) with the idle adjust screw all the way out. A vacuum leak I'm sure. Great post, thanks for the info. I'm going to run some injector cleaner through it tomorrow, see if that makes any difference. If not, I'll send them out to be cleaned professionally. Cannot afford a new set period at the moment. The car has under 120k miles, the injectors should still be OK I would think.

I have an 86 gxl and im getting the same symptoms, low ruff idle, and poor mpg, i calculated about 15 on my last tank so i bet my injectors are worn out, considering my car has 160xxx on it