1994 Honda Civic Stumbling

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I have a Honda Civic LX 1994 that stumbles on acceleration. It seems to be worse when it is warm. It feels like only one of the cylinders misfires while accelerating, making the car lack power and vibrate, and
once the RPMS get high enough, at around 3000RPMS, the misfiring dissapears. Nonetheless, it is a combination of the engine load versus the RPMs, I can get it to stuble at any given RPM setting, if I press on the throttle pedal too hard. Here are the parts I have now replaced:
Entire distributor, including the cap and rotor. Spark Plug Wires Spark Plugs Fuel Pump and fuel strainer Fuel Filter Fuel Rail All 4 Injectors Fuel Regulator Air filter
I have checked the TPS with a multimeter, 0.5V at closed throttle and 4.5V at full throttle smooth transition in between. I have tested the MAP sensor with a hand vacumm gauge and a multimeter, sensor is all nominal but here is the chart if anyone cares to look at it: 0psi    2.80V 5psi    2.34V 10psi    1.86V 15psi    1.39V 20psi    0.96V 25psi    0.52V I have tested the fuel pressure, pressure is normal. I have tested the O2 sensor with a multimeter and propane torch. Again it seems to be normal. I have also disconnected it from the car and driven it without the lambda sensor, still the car behaves the same and stumbles. I have checked the timing with a timing light, and the timing is right on according to specification. The ECM reports no trouble codes. The ECM reports no "Check Engine" light while driving, although the car stumbles wildly. I am running out of ideas... can anyone think of anything else I should check or that could possibly be the reason for the car stumbling.
- JR
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A few more basics to check or do:
-- Dump a bottle of Chevron Techron fuel system cleaner into the car's near empty fuel tank. Fill with gas.
-- Check the ignition timing.
-- A bad O2 sensor will not necessarily set off a CEL on a 1994 Honda (pre-OBD2). Since the problem occurs after warmup, that makes me especially suspicious of the O2 sensor. https://www.automedicsupply.com/ has the best prices I've seen for OEM O2 sensors: About $69 altogether. I have used them. Good, prompt service. Maybe not a bad investment for a car this old.
-- Purge cooling system thoroughly of air. Follow the manual's instructions. Getting the fan to come on may take as much as 40 minutes of idling. If various ECM sensors aren't cooled properly, then this may cause erratic behavior.
Are all the ignition parts (plugs, wires, distributor etc.) OEM?

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Like I said, the O2 sensors tests OK. I have unplugged the sensor and driven without it, only to experience the same problem. The O2 sensor has only 20,000 miles on it. It was put in six months ago; Bosh made. All other replaced parts are OEM.
- JR
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snipped-for-privacy@sgintl.com wrote:

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What's the mileage on the vehicle? Did this problem show up after your last Timing Belt replacement? Were the valves adjusted at the same time?
P.S. Almost everybody agrees that Bosch O2 sensors shouldn't be used on Hondas :-( Same for their plugs.
'Curly'
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I have personally rebuilt the engine... the problem did not show up after the timing belt replacement, which happened at the time the engine was replaced. All of the major compenents in the car were changed in 20,000 miles ago, along with the entire motor. The car has been running just great until now.

Even then, unplugging the 02 sensor, the car experiences exactly the same problem I have described.
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snipped-for-privacy@sgintl.com wrote:

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How far back (days / months/ miles) does the term "running just great until now" go back? Last fill of gas?
After you got the engine re-installed, did you find that you had to move the distributor about 10 degrees to get the ignition timing correct? If so, I'd guess that the cam timing is out, but the ignition timing is correct(ed). Do you have to put the jumper on the ECM connection when you time a '94, like you do for the newer (OBDII) ones?
Is the new engine an exact match to the car, or did you import the ECM as well?
'Curly'
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Running great until 100 miles ago. Running well for 20,000 miles after rebuild. I didn't have to do anything special except point the gun and align the lines on the crank pulley with the alignment mark on the timing cover. You DO have to jumper the connections with a a jumper wire, and you have to wait until the engine has come to normal temperature to do the timing. The engine is an exact match to the car.
- JR
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snipped-for-privacy@sgintl.com wrote:

How about a compression test?
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Interesting answer... I never thought the problem could be mechanical. I will run a compression test. I will let you guys know.
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snipped-for-privacy@sgintl.com wrote:

I don't know if a bad ring would do that or not.
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Alan wrote:

I doubt a ring would cause the problem; though I will certainly check. The car does not burn any oil.
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Alan wrote:

i second that. unless this is something really rudimentary like a vacuum hose has fallen off, a burnt inlet valve is a good candidate for the symptoms described. it should be very distinctive on cold cranking though - instead of 4 even compression cycles, one will "wow" much faster than the rest.
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What causes an inlet valve to burn? I am at work now, but I will run a compression test later this evening. I will let you guys know the results. All of you have been very useful... at least I feel I am going somewhere trying to diagnose the problem.
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The usual cause is inadequate valve lash. If the rockers are too tight the valve will leak and the combustion gasses will torch a channel in the valve. That should show up on the compression test, although it may not be shocking yet.
Mike
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I am curious as to whether he has gauged the voltage from the coils at the spark plugs. If I've read correctly, he can get the misfire to occur at will by increasing throttle pressure relative to extant engine RPM--in other words, putting enough load on the engine--to the point of the voltage being insufficient to ignite charge. For what it's worth, it is also within the realm of possibility that the engine controller itself it the culprit, hence no MIL (check engine light). Note that I'm not experienced with Hondas in particular, I'm just throwing this out as my 2. New to this group (looking at a Honda in a car lot)! :0)
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I had an old K car that would misfire under a load. I would briefly test it by one foot on the brake and the other on the gas. The culprit was a bad or weak coil.
James Goforth wrote:

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Considering that Honda/Acura autos are known for failing ignition coils,this would be a good place to start.
--
Jim Yanik
jyanik
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That's not dispositive.
I personally would not yet eliminate the O2 sensor. That it's Bosch certainly makes me suspicious.
Two cents.
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The problem occurs at all times, it just happens to be MORE pronounced when warm than cold. As I have said before, I have cheked the timing and it is right on the spot. Following your suggestion I have gone through the air purging on the coolings system (which did not need to be purged by the way). No change. Thank you for your suggestions.
- JR
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Elle wrote:
.......A bad O2 sensor will not necessarily set off a CEL on a 1994 Honda (pre-OBD2). Since the problem occurs after warmup, that makes me especially suspicious of the O2 sensor. https://www.automedicsupply.com/ has the best prices I've seen for OEM O2 sensors: About $69 altogether. I have used them. Good, prompt service. Maybe not a bad investment for a car this old.
Elle, I went to this site, & an O2 sensor for my 94 VX is $289.99. Before I spend this much money, what symptoms would I be seeing?
.....Purge cooling system thoroughly of air. Follow the manual's instructions. Getting the fan to come on may take as much as 40 minutes of idling. If various ECM sensors aren't cooled properly, then this may cause erratic behavior.
I'm following the manual for a 92 Civic, & reference a 95 PDF which is identical. I've pulled & cleaned the reservoir, then replaced. It appears the radiator drain plug turns counter clockwise. Any hints on how to get it to move without breaking it?
I started the car, put the heater temp control lever to max heat, then shut off the engine. No mention of whether or not to have the fan running while I purge the system. Any ideas? I'll top up the fluid today, then once I can get the drain plug to move, do a complete refill.
I've tried bleeding before, but had the cap completely off. Manual says to put it on to the 1st stop. I was still getting the erratic idle at a stop with this approach.
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