upnsmoke

Hi all. I haven't been on in a while as I've been without a Civic for a while. I recently acquired a 91 DX hatch with a throttle body injected D15b7 from a friend and one of the first things I set about doing was an mpfi swap. All has gone well so far until now. The motor starts easily, seems to rev up freely, and idles smooth. Under light throttle, however, the engine will rev up and down as if bouncing off a rev limiter. I drove the car briefly and under light load it surges badly. The only CEL code is a 14, which, according to what I have read is for an IAC. I am currently not running an IAC. My intake manifold is from a D16Y8 with an automatic and I eventually intend to drill and tap the manifold for a manual style IAC mounted on the rear of the manifold, just haven't gotten to it yet. I made a block off plate for the IAC which normally sits under the throttle body. I did plug a known good IAC into the stock harness just to see if the CEL would go off and it did not. The code comes back every time the engine is started. I don't know if my issue is related to that. The motor seems to be running pretty rich right now, especially at idle. Free revving or under load the motor responds well to moderate or heavy throttle.

What I have done so far in attempts to diagnose:
Check TPS: seems okay, voltage has a smooth sweep from .5v closed to 4.5 at WOT
Checked MAP sensor: voltage appears in range around 3v key on engine off and .95v idling
Set ignition timing to spec
Checked for vacuum leaks and found none (trying to find my stupid vacuum gauge and will use that to double check when I can)

Replacement parts and maintenance done so far:
D16y8 head gasket installed
D16a6 cam installed and all rockers adjusted to spec
Timing belt, water pump, front seals, used an adjustable cam gear to degree the cam where it is supposed to be (the half tooth off thing with the d16a6 cam in the d15b7)
New intake manifold gasket, new cap, rotor, ngk plugs and wires, denso o2 sensor with less than 1k miles

My ECU is a pm6 and I don't currently have another one to plug in as a test. I have been over my wiring as thoroughly as I could and have yet to find anything wrong. My distributor is a used td02u and I don't know if I may have an issue there, or how to tell if anything is wrong with it. Is it possible the distributor could have a problem with one of its sensors and not set a CEL? The engine ran fine with the dpfi and I believe it to be mechanically sound. I attached a fuel pressure gauge. Pressure is range and regulator appears to be doing its job. I'm kind of at a loss as to what to do next and I appreciate any input anyone can give as to what is most likely causing my problem or what I should check next. Sorry for the long post. Thanks to anyone who reads and please let me know if I have forgotten to include any relevant details.

upnsmoke

Hi all. I'm still playing with this and have a couple more thoughts on it. I never planned on keeping the used distributor and had ordered an Autoline reman, which came in today. Unfortunately I am still having the same issues with it installed, so I guess the problem was not the distributor. One thing that has me intrigued was, just for the hell of it, I switched the two wires to reverse the operation of the tps sensor, even though I knew it was correct as installed. Strangely enough, the engine idled fine and the surging was gone. It would not take full throttle or anything close to it, which in itself does not surprise me. I can't imagine why it would run better at and near closed throttle, though. At this point I am going to look at the injectors next. They came with the junkyard manifold and I have never had them out, so I am going to clean them and inspect the o rings. I wonder if the injectors could be dirty, causing a lean condition that is more noticeable at low throttle inputs, or maybe the injector seals are leaking, causing a vacuum leak. I thought the motor was running rich, but at this point, who knows. I would think the tps being backwards would make the ecu think the throttle was open more than it was, at least at or near closed throttle. Being as it seemed to improve the way the motor ran, I can only think that I may have a fueling problem. I have installed a permanent gauge on the fuel filter outlet, so I know that side of the system is correct. Next logical step for me is the injectors. If they are fine, I guess I'll have to start looking for another computer. Sorry for rambling. I'm just thinking out loud at this point. I still appreciate any ideas anyone else has. I'm really looking forward to driving this car.

upnsmoke

Well, I'm back to square one and more lost than ever. The motor started running terrible with the new distributor and I ended up with a code 4 and 8. I put the used distributor back on and it's the same as it was when I first started it. Idles smooth, free revs fine, just surges when trying to hold steady rpm. Only code with used dist is 14, which comes on as soon as the engine is started, regardless of IAC being plugged in or not. I pulled the injectors, tested them on the bench best I could. All work fine and seem to have a good spray pattern from what I could tell visually. Cleaned them anyway while I was at it. O rings look fine, still soft and not ripped. I have no idea why the reman distributor is acting weird. I have always gotten good products from Autoline and don't mind paying a premium for their stuff, but in this case I have no idea what is going on. While I was at it I pulled the valve cover and checked the rocker lash, which is still where I set it. The new (used) cam looks fine and there is no ticking or unusual noise with the engine running. I lent out my vacuum gauge and will be getting it back tomorrow, so I'll see if that tells me something useful. My ECU is a completely untested (other than in my car, which is an unknown in itself) junkyard find, so perhaps that in itself is a problem. Still doesn't explain the distributor anomalies, which is another variable I will have to address. I have ordered a known good ECU that I will try when it gets here. I hate to throw parts at a problem, but at this point I am running out of variables to eliminate, and I am trying to have this car finished as soon as I can. While I am waiting for the ECU to arrive I will review my wiring, test, and redo some connections just to be absolutely certain that is not the source of my problems. Wish me luck in the meanwhile, and I'll update with any new findings. Hopefully someone can chime in with some thoughts. If nothing else writing all this out has really helped organize my own thoughts, and that in itself is very useful to me.

To summarize what I have so far:
Engine specs: D15B7 stock other than D16Y8 intake manifold, D16Y8 head gasket, very mild port and polish, D16A6 cam, ~150k miles
Management: PM6 ECU, high impedance injectors with no resistor box, firewall type MAP sensor, (throttle body mounted was bad, waiting on new one)
Things checked (and rechecked): cam timing, ignition timing, valve lash, no known vacuum leaks, TPS and MAP sensor, fuel pressure good, injectors clean and functional, fresh secondary ignition, coolant full and bled, o2 sensor almost new, test pipe installed to eliminate possibility of clogged cat

If anyone can think of anything that might help me, anything I am not thinking of, am forgetting to check, or am mistaken about, please chime in and let me know what you think. I really do appreciate it.

upnsmoke

Hi again all. Seems I have fixed my problem and the ECU was the main offender. I have a new TB mounted MAP sensor, a new TPS. I changed the TPS as my previous one seemed to work best in a certain range and I thought it may have had a bad spot that may not have been apparent with my cheap DVOM. I also installed a new throttle body mounted MAP sensor as mine was known bad and I had been using the firewall mounted part that came with the car. With all that done the motor was still surging at light throttle. I installed a known good computer and now have no CEL (had been getting a 14 code as soon as the engine was started before). The engine starts readily, idles smooth, and most importantly has no surging at steady rpm and light load. All seems well now, so I guess the ECU was the problem. I have to finalize my wiring harness and will hopefully be able to start driving this car again soon. I'm really looking forward to the extra power. Maybe now I won't have to floor it all the time just to keep up with traffic.