Strange richness, Choppy Idle after 1gina2g swap.

[my_gst95]

I purchased a non-running 95 2g which ran out of oil. Perfect time to do a complete 6bolt swap. Mind you I've done this swap procedure on 3 cars before without problems. But since I do not know the history of this cars past, I'm having trouble diagnosing.

Problem: Rich idle condition with exhaust note popping. Free revs fine, but even holding at a higher RPM, popping still audible. It sounds as if there is misfires happening. Sparkplugs are black, verifying rich condition. Currently no CEL.

Procedure: Complete 6 bolt swap. Using a 95 ecu, so I used the usualy Magnus info on using a 90 CAS, swap injector wires at the ecu, swapped firing order since this is a 95-96, nothing I haven't done before. Extended the Power Transistor to the firewall. Extended the MAP sensor to the firewall. 1g coil bracket with the 2g coils switched around.

Inspection: Compression is 150 accross. I had a 6bolt from a donor 1g and it had 150 accross before I pulled it out. After I pulled it out I had the head completely rebuilt, and the shortblock freshed up with every gasket from head to toe. So 150 in, 150 out. Mechanically the car should be solid at this point (assuming). Swapped MAS's, problem remains. Disconnected front 02 sensor, problem remains. Disconnected rear 02 sensor, no change. Open exhaust (catless), no different.

Since I don't have a 2g logger to see what the car is doing, I have to work somewhat in the dark in the mean time. So, what would cause the car to act like this? I tried grounding the Power transistor in different locations, but to no avail.

 

[my_gst95]

Just tried disconnected the TPS to mask the Throttle position to the ECU to see if it was just the usual Random Misfires and still no go. So it has to be something with the firing order/spark. I tried routing the wiring to the 1g CAS under and about the intake manifold, same results. Perhaps this is a faulty CAS? I'll try purchasing a black-top 93-94 this week to see if it makes a difference. Any ideas/help still is appreciated.

 

[Kevin Jewer]

First off, excellent post! Very well structured. There is hope left for the world after all...

Trying to diagnose these things without a logger is a real crapshoot. I'll just highlight a couple things that have bit me in the ass over the years. In my experience, a CAS will either work fine, or just straight shit the bed. It may be intermittent, but it won't run when it's acting up. A bad coolant temp sensor or broken wire (perhaps it wasn't hooked up in the swap) will read -40F at the ECU and will run very rich. The ECU also will not enter closed loop fuel control in this condition, possibly explaining why unplugging the O2 sensor had no effect. A dead front O2 sensor (doesn't cycle at idle) will also force the ECU to remain in open loop or continue to add fuel trying to get it to cycle. This will be allowed to continue for quite some time before it will throw the CEL for it!

Sounds like you hit most of the usual things, so these are what I would work with next. Does the car run well or have you only got as far as idle at this point?

 

[my_gst95]

Thanks for the reply. Good point on the CAS, as it simply sends the signal to the ecu so it would either work or not work at all. Coolant temp sensor is an idea, but I've got the car to operating temps according to the stock gauge cluster, and the fans do kick on so I know it's working. This one I guess will have to wait for a logger to verify the -40F as you say. But I'll double check on the wiring. As far as the O2 sensor goes, maybe I'll hookup an SAFC today to verify the sensors are getting a reading.

The car drives as if it is in limp mode or constantly popping/misfiring. So I guess it's 'somewhat' driveable. I just had a thought. Could the timing be off? Stock cam gears seem to be pointing to each other as they should, maybe the crank is off a tooth? I doubt that it is, simply because the compression set I got is almost the exact same as before I had the engine rebuilt. Wouldn't the compression set numbers be off/lower if it was off a tooth or more? Worse, wouldn't that also equate to bent valves as this is an Interference type motor? Yeah, I just can't imagine this motor being off in timing, but I guess it may. I'd like to reserve checking this last as I hate disassembling the belts and Harmonic Balancer .

Thanks again,

 

[my_gst95]

Ok now this is starting to piss me off Pulled the timing assembly off and all the timing marks are spot on as far as I can tell. I can tell it's firing correctly also because when I rev it up it revs smoothly and sounds like it should aside from the few pops here and there. I hooked up a logger. I can't see anything wrong. Timing shows 11°, Coolant temp climbs to 198-200° before the fans kick on as they should, Airflow increases with throttle increase, TPS responds correctly. Only thing concerning is that the O2v's didn't ocolate when car reached operating temps. Even if it is a bad O2 sensor on bank 1/2, I doubt it would cause the car to idle like since the car is in open loop anyway when warming up.

I just noticed that as the car is having a hard time idling, i can hear what sounds to be air being released/sucked abouts the area of where the nipple is on the Intake manifold leading to the PCV valve. Reving the car up and the sound goes away. I tried capping the TB nipples and that nipple going to the PCV, I can still hear it. So now I'm thinkig it's a possible vacuum leak. Boost gauge verifys a loss in Hg. Can I test for a vacuum leak via a boostleak tester connected to say the TB elbow?

Manny

 

[olmytsi]

Yes you can. You might as well boost leak test the entire system.

 

[my_gst95]

Found it. I can't believe what it was. The throttlebody-to-intakemanifold gasket was upside down....yeah...upside down. It was letting air out of the bottom of the throttle body. I didn't even know it was possible to put that gasket in wrong . Hope this helps people with similar problems.