Misfire under load, and low RPM

[MattZig]

My 92 1.8 MX3 has a pretty bad misfire. It's way worse when warm, and when sub 4,000 RPM, and especially under load. What happens is that I'll be accelerating and the car will jerk, and then jerk some more, and more and more and you can hear a defenite miss in the exhaust. At idle, the car will sometimes shake so badly its embarrasing. When viewing the back of the car from the outside, you can definitely hear the exhaust misfire and the condensation stop for half a second each time.

Above 4,000 RPM the exhaust note changes (gets a little louder) and the car runs fantastic up until redline. Doesn't miss a beat. What could be causing this? I've replaced spark plugs, the ignition wires are all in spec, cap & rotor recently changed, fuel filter changed, intake gaskets recently changed and checked for vacuum leaks (there were none).

I'm thinking it has something to do with vacuum. I had the same symptoms only 3x worse a month ago and fixed it by taping up a hole in the air boot letting un metered air in. But I'm positive there's no more holes and its still doing this.

[marcdh]

Could be the coil in disty starting to give up, happened to my daily driver recently. See if you can borrow another disty locally or alternately search for the HEI mod.

[Ryan]

This is very likely a timing issue. Check to make sure its in time. If you can't get it to time properly, the belt jumped. This happens, its very possible. Crap falls into the covers (maybe a piece of the cover itself) and down by the crank gear and boom, a tooth or two off.

[MattZig]

Can't be cam timing, and I don't think its ignition timing either. I just did a test today:

When the engine is cold, at 50 KPH and under 2000 RPM in 5th gear, I can hold it at any throttle position with barely any misfires going on. No bog, just poor acceleration which is normal.

When the engine coolant is in the middle, ie warm, and I try the same scenario, I get all kinds of bucking, misfiring, and a definte missing sound from the exhaust.

Thing is, at high RPM at even high load, it runs fantastic. Like 4-5k+. Wouldn't that rule ignition out? I'm thinking more along the lines of vacuum, but why only when warm? O2 sensors maybe?

[Ryan]

Can't be timing? Why is this, because you said so? Understand the system: The timing advances more and more with throttle and RPM. If your timing fell retarded, once you hit a certain spot in the RPM band you might hit an acceptable part of the timing map, and it will run great then. Why my black car jumped two teeth, it ran like absolute s--- below 4k, and great over 4k.

Borrow a timing light and check out your timing.

Does your car throw any codes?

It might be distributor failure, that does some whack things that aren't always the same for everyone, but along these lines. Try another disty.

There is a difference between "diagnostics" and "guessing". Its a pet peeve of mine when people come on here for "advice" when they're really just looking for someone to agree with their diagnosis, and completely ignore the legit advice.

[MattZig]

Hold up - are we talking cam timing, ignition timing, or both? The reason I said it can't (or rather, probably isn't) cam timing is because with a tooth or more off, you would think that it would run like crap all times - hot AND cold. Mine only does it hot. How would that be cam timing? Ignition timing, maybe - which I said. But cam timing? Not likely.

[Daninski]

Point a timing light at the underdrive pulley and see where your at. If it's around 8-10 your gold if it's way out that'll tell the tale.

[Ryan]

The electronic timing changes more than one tooth, and it changes as a function of temperature, among other things. It is very possible. Dan is right, get a timing light on it, and if you can't adjust the disty enough to get it in time, its probably out mechanically.

[mikeinaus]

could this be caused by a faulty coolant temp sensor?

[MattZig]

Ok the misfiring is fixed. Replaced the ignition wires with junkyard ones, and taped up my airbox hose which was apparently quite cracked. Absolutely no more misfiring at all. New airbox boot is going to be ordered asap.

It still feels like VTEC is kicking in at 4500 RPM. Seriously almost no torque and then it just takes off like a rocket. Could be ignition timing for that - I'll have to check. But for now I'm happy. Thanks for the suggestions.

[unclegreg]

It still feels like VTEC is kicking in at 4500 RPM. Seriously almost no torque and then it just takes off like a rocket. Could be ignition timing for that - I'll have to check. But for now I'm happy. Thanks for the suggestions.

Sounds like one of your VRIS solenoids are not functioning properly.

[MattZig]

I remember an engine code telling me this awhile ago. There aren't any codes stored right now though.

What's weird is that even the exhaust note changes like VTEC does. It'll be revving up slow, hit around 4500, then instantly get louder and I'll get 50% more torque. It's definitely not misfiring at all anymore, so it's not like its gaining a cylinder. Must be a timing or exhaust issue. Could be that my cat is plugged. Could be the rings too, I have to do a compression test.