Idle Problems in CRX after trans swap - Fuelly Forums

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Old 03-04-2009, 12:11 PM   #1
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Idle Problems in CRX after trans swap

I've been fighting this problem on and off for about a month now (mainly because I don't drive the car much during the winter). So far the guys at honda-tech and hondaswap haven't been able to help... I'm hoping somebody here has a few ideas:

The car: 88 Honda CRX DX. Originally AT car, recently swapped to stock DX trans (ECU also swapped).

The problem: After the swap, the car seems like it never comes off the warmup cycle. Idle is always high (1100-1700 depending on who knows what), but will drop to 750 if I blip the throttle like you would with a carbureted car to knock it off the fast idle. The next time I rev the engine it goes back to the high idle. It also pulses between 1100-1400 on rare occasions (only twice in the past month). The RPM also seems to drop more slowly than it should (or not at all) between shifts as if the ECU is keeping it artificially high.

What I've checked:
1) engine codes - none
2) coolant system - properly bled and filled
3) ICV - cleaned
4) linkage - inspected and lubricated. No binding or other problems. Vacuum actuators, appear to be working properly, but it's hard to tell for certain.
5) Sensors - need to test, but can't find anything to compare the values to.

I'm down to thinking that it's either a bad temp sensor, or maybe a bad O2 sensor that's spoofing the ECU. Aside from that I have no ideas except maybe that there was a wiring difference between the auto and manual ECU that needs to be changed. All I swapped was the Neutral cutoff and backup light wires. Everything else was just plug and play (or maybe I'm wrong on that?).

Any help or troubleshooting advice? Now that the weather is warming up again, I'd like to get this resolved so I can drive the car on a regular basis.
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Old 03-04-2009, 01:25 PM   #2
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I'm guessing you swapped in a MT trans? Maybe the TPS is different between the AT and MT models(they sometimes are) and the MT ecu isn't getting the right readings. Often, vehicles will open the IACV(Idle Air Control Valve) when you aren't at idle to increase coasting distance. But, that also makes me now think that the speed sensor might be the culprit. If it thinks you are rolling it might keep the engine in a high-idle. But that's harder to explain then a bad TPS because something would have to be pulsing to the ecu at idle making it think there was movement, which could be cross talk from the ignition system but that kind of thinking can get you off on a whole different tangent.

Another thing to consider is a bad ecu or the pinout between the two ecu units weren't exactly the same causing any of the above issues.
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Old 03-05-2009, 07:20 AM   #3
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Thanks DK,

Yes, I swapped to a MT (sorry, I didn't specify that in my first post). I'll I'll check with the dealership and see if the TPS is different.

Any idea where the speed sensor is on the car?

What about adjusting the TPS? Is that possible, or are the ranges completely different?
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Old 03-05-2009, 07:31 AM   #4
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The speed sensor is plugged into the top of the transmission. Honda tps's for dpfi and mpfi have the two outer pins reversed, and when I used an mpfi tps sensor on my dpfi 91 hatch std, I had to reverse the outermost pin on each side. Your situation may be different. I would not think there would be any diff b/w 5spd and at in regards to the tps sensor.
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Old 03-06-2009, 06:56 AM   #5
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Huh? The only sensor I remember in the top of any of the manual transmissions (I ended up w/ two bad ones before I got the one I installed) is the reverse light switch. Maybe the problem is a missing speed sensor? I dunno but I'm grasping at straws at this point.

Any chance you can pop your hood and snap a photo of the speed sensor for me? At least then I can really home in and take a close look to see if there's a spot for the switch.
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Old 03-06-2009, 07:16 AM   #6
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typically auto trannied cars idle higher than manual trannies so there could be a sensor issue,

also loose/missing/cracked vacum lines can cause high/erratic idle too
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Old 03-06-2009, 09:26 AM   #7
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You don't have a speed "sensor" on your transmission. Your car has a cable operated speedometer - it's purely mechanical, aside from the small electronic bits at the gauge cluster.
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Old 03-06-2009, 01:51 PM   #8
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Yeah, a bit of googling revealed that there's a VSS on the back of the instrument cluster. I'm guessing that if it was good when I had the AT in, it's good now since it doesn't appear to be transmission specific.

Vetteowner, I've checked the vacuum lines carefully and they're all nice and pliable. None cracked, cut, or disconnected.

I have some testing to do, but it will have to wait since I'm getting ready to leave for a hunting trip and will be out of town till next weekend. But I found a website with the sensor values and a lot of other good information for this vintage Honda.

http://geocities.com/chipman_13/ecu.html#codes

Honestly, I'm beginning to think that I might have a bad ecu (it was tested before I got it, but it's also the only other piece of the puzzle that got changed out w/ the transmission. Maybe I'll pick a replacement up off ebay and see if it makes a difference.
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Old 03-06-2009, 02:15 PM   #9
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I'd recommend getting the sensor values for the car the ECU was originally in and then spending some quality time with your multimeter under your hood before calling that ecu dead.

BTW, what did it come out of anyways?
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Old 03-09-2009, 09:10 AM   #10
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I'm going from memory now (I bought the ecu like 7 months ago, and I didn't pull it myself), but I think it was from an '88-89 Civic DX. It was a DPFI car for sure, and it had supposedly been tested. Everything should be the same as my CRX, but If I can find the stock ecu number for an '88 CRX DX 5 speed I'll compare the two to confirm that they're the same. I guess it's possible that the ecu is compatible (more likely than that it's failed at least), but until I get a chance to do more on the car (probably weekend after next due to my schedule) I'm out of new ideas.
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