I posted this in my build thread as well, but figured here might get more traffic. The engine is a S5 NA that I bought from Kevin Landers. He had previously rebuilt it, sold it, got it back and then resold it to me. I swapped it into my 85 using an FC in tank style fuel system and it's all wired up to run the stock S5 ECU. Those who know me will know that I've done several S5 swaps before (always T2's though, never NA's) and have had great success.

This one though has some issues and I'm stumped. The engine has 8000 or so miles on it and actually starts up very easily, cold or hot doesn't matter. The engine itself is obviously strong. I actually managed the get the car smogged with no drama, and put a few hundred miles on the car - it drives pretty well.

Here's the problem - it's got a VERY rough idle, and hunts around quite a bit - more so with a load on the engine like the head lights or fan etc. I have swapped throttle bodies to one that has no thermal wax stuff on there, to remove that possibility (the FC warmup crap on there is notoriously bad, I have it removed on my T2's as well, always starts fine) Adjusted the TPS a few times. I swapped basically every sensor on the engine, changed injectors, and removed/resealed the upper intake manifold a few times. I checked fuel pressure and volumne -both within spec.

I actually went as far as pulling the engine back out this weekend since I am fed up with it - this is where it gets interesting. When I removed the header, I noticed that both exhaust ports are vastly different colors. Here's where I need help. I know that the front is obviously running richer than the rear, but which one is correct? Typically a hunting idle would be from it leaning out too much. Is the rear one in my case too lean? I'm trying to narrow down what the problem could be. My next step now that the engine is out is to strip all manifolds off of it and start fresh with new gaskets etc to alleviate the possibility of a vacuum leak, but those exhaust ports have me wondering what the problem actually is...

I'll be the first to admit that I'm much better at metal and body work than I am engine diagnosis, so any input from the engine guys is appreciated

-Sean

 

Could be one of two things or a little of both. The rear primary injector could be a bit clogged giving a lean condition. Or the your brake booster has a leak allowing more air into the rear rotor where it's drawing its vacuum.

My car has a leaky brake booster so that's why my rear sleeve looks lighter colored and my idle has a small vibration. A quick fix for me was to add a brake booster fitting to the front secondary runner. I'll eventually get around to swapping in another booster but for now this should work.

Your problem is probably an injector. But try pulling off the brake booster hose and blocking it to eliminate it as a possible vacuum leak.

 

I'll give that a try, although I did neglect to mention that the booster that was in this car was basically non operational it leaked so badly. I replaced it with a pretty rough looking used one i had. No guarantee that this one is leak free, although it did help the pedal feel quite a bit, so it's at least "better"... Worth a try to block it completely and see what happens.

I did try changing injectors (to ones from a parts engine, so again, no guarantee they are any good either) and there was no change. The connectors to the injectors looked ok, but that's worth a revisit as well.

 

usually a good running engine will have light brown or off white exhaust ports, black is bad.

 

That's kinda what I had thought - my other engines are usually a tan off white type color.

What do you think is making the front port on this one look so black?

 

Looks like soft carbon soot, which is usually a rich-condition symptom.

 

Yeah that makes sense. This car is driving me nuts...

 

does it have a license? /rimshot

 

^har har.

So my next question - the slighly too light colored rear exhaust port is probably a vacuum leak, or crappy injector like Jeff said. What's causing the sooty/rich port on the front rotor? Or rather, what should I be looking for on that?

 

You know, the front injector could be a bit sticky or partially stuck open, in addition to a possible brake booster leak on the rear rotor. Try swapping primary injector positions and block off the booster.

 

Here's what I'm hung up on, I swapped a spare set of injectors (from a unknown engine, thats been sitting for a long time) in last week and the symptoms were exactly the same. Seems unlikely that both sets would do the same thing though doesn't it? Or maybe all the injectors I have are crap (very possible).

 

Or it might just be a leaky booster. That might be all it is, and your engine is trying to compensate. EFI can do that where as carbs won't as much.

 

compression?

 

Ok I'll investigate further, at least now I've got some focus on where to look, thanks guys.

Compression seems unlikely since this has under 10k miles on fresh seals (according to Kevin, I have no proof) but it could be worth a quick check.