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Old 10-25-2018, 05:50 AM   #1 (permalink)
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Default Bouncing Idle - Bogging under Load on New Build

Hey guys,

So a friend and I did a D16A6 swap into a 1991 CRX HF. The engine was totally complete, except for the air intake and throttle body which came from a Z6.

When we start the car with the D15 ECU, it idles OK, but we don't want to run it under load, as the fuel mix will probably be incorrect.

When we start the car with the D16 ECU, idle bounces up and down, evening out slightly after it warms up. Under load, it bogs down in the 2-4k RPM range, and then starts behaving more normally in the higher revs.

The car has one of those AEM style air intakes, and I'm hearing a hissing from the filter area at idle. I've been reading that this is normal, but I don't remember that sound when the D15 was running in it, though I could be wrong.

- Mechanical timing is correct, ignition timing is correct, valve lash was done. I'll recheck all those things, but fairly certain they are right.
- TPS is from the D16 engine, but is adjusted correctly.
- We've bled the coolant
- Plugs are new, but I'll pull and check the gap.

On my list for the weekend is to recheck ignition timing and TPS, maybe swap the TPS out for the old one from the D15, pull the IAC and bench test it if I can, and check for any obvious vac leaks.

Another concern is that we are using the injectors from the D15, which are apparently 190cc, while the injectors that the D16 ECU is expecting may be 240cc?

Just wanted to put this to the forum and see if there were any obvious suggestions, or anything obvious we are overlooking.

Appreciate the help.
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Old 10-25-2018, 02:35 PM   #2 (permalink)
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It does sound like a tuning issue. I'd be tempted to try the D15 ECU at least until the car gets up to temperature.



[Disclaimer: I've never done a swap like that.]
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Old 10-25-2018, 05:51 PM   #3 (permalink)
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I have seen these kinds of symptoms during a swap, especially across electrical architectures like going from OBD0 to OBD1/OBD1 to OBD2, where ECU pins, chassis connector terminals, engine harness to chassis harness pins/terminals get out of alignment and engine component pins lose their correct ECU pin correlation.

If the engine runs, it definitely sounds like pin-to-component alignment is very close to completely correct. Double and tripple check this, as no amount of tuning/component replacement will remedy incorrectly routed wiring!

Some things to be on the lookout for are small things like ECU circuits being one pin off in a similar connector from one ECU/chassis to another, etc. especially if the swaps both share electrical architecture. It is a common occurrence to have pins move one connector hole up/down/left/right from the same connector in another ECU/chassis.

The only way to know for sure is to plot all circuits from both cars and their respective ECU's across each other, making sure that all the pins on the swapped ECU get what they need from the car as well as making connections to all required components at the engine. This can take a couple hours to verify, but it's worth it to find anything that might be wired incorrectly.

If circuits are not aligned, you will have odd symptoms. It could be as simple as depinning connectors and moving the wires around within a connector so that everything matches the schematics.

If the swapped engine ran fine on its original ECU in its original car, and only now are you having problems with it trying to run right, definitely look at wiring/terminals/pins not matching up because it will cause funky drivability symptoms like that.
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Old 10-25-2018, 05:59 PM   #4 (permalink)
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Quote:
Originally Posted by kyngfish View Post
Another concern is that we are using the injectors from the D15, which are apparently 190cc, while the injectors that the D16 ECU is expecting may be 240cc?
Always make sure the injectors for the swap are what gets installed. Using smaller/larger injectors vs what an ECU is originally programmed for will definitely cause weird driveability problems.

If the D16 is what is in the car right now, put the D16 injectors in if using the D16 ECU. You need all of these to match to have a chance at good running conditions.

It sounds like you are definitely on the right path, questioning these kinds of things. If the swapped engine is staying stock, getting it to run right should be as straightforward as bringing back all the stock things the swapped engine needs (this also includes correct wiring/pin/terminal routing for the swapped engine).
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Old 11-05-2018, 06:54 AM   #5 (permalink)
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Just a quick update - in case someone gets into a similar issue with bogging under load.

After a little research, looks like the injectors on the D15 in 1991 were the correct 235cc, so we are good there.

Fiddled around with the car on Sunday, and after pulling some sensors and messing with vacuum lines, we decided recheck the ignition timing with a timing light. The annoying thing is that the way the three hashmarks align with the sights along the plastic timing belt cover mean that when timing is ON, they are obscured by the sights, so basically, we had to pull it off-time and slowly inch back in, guessing we were on more or less when the hashes disappeared. If there's a better way to do this, let me know.

After getting the timing a bit better, I took it out for a spin and it was night and day, we got solid acceleration in gear and it felt pretty smooth all the way up the tach under medium and wide open throttle. We still got an occasional slight hesitation, under light throttle, but I'm guessing the timing still isn't perfect and the big hiss I'm hearing at the throttle body is a vac leak that probable needs to be taken care of.

That said, the engine had been running for a fair amount of time by the time we took it out, so we'd like to take it out cold before claiming victory. Still some minor issues with idle and that slight hesitation under light throttle but progress is progress and we feel confident it's a step in the right direction.

Thanks again for the pointers guys. Learning a lot.
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Old 11-05-2018, 12:14 PM   #6 (permalink)
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Quote:
Originally Posted by kyngfish View Post
... slight hesitation under light throttle ...
Check the points inside the dizzy cap. It's free to check and can cause this problem. The corrosion can build up on the points and prevent sparking. Use a flat head screw driver or a small flat file to chip the corrosion away.
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Old 11-05-2018, 12:24 PM   #7 (permalink)
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Quote:
Originally Posted by EFB055 View Post
Check the points inside the dizzy cap. It's free to check and can cause this problem. The corrosion can build up on the points and prevent sparking. Use a flat head screw driver or a small flat file to chip the corrosion away.
So this might make sense, because I'm also getting an occasional backfire there under light throttle. Usually when accelerating from a stop. I would have expected with a vac leak that the mix would be lean, and so the backfire didn't quite fit.
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