1988 GTU

The car has done this since the day I got it. While idling, it'll make a muffled pop-noise from around the catalytic converter area, occasionally; more-so when it's cold than hot.

Also, when cruising down with my foot off the clutch and the car in gear (2nd gear is common when pulling up to the house), right as the rpm's drop below 1500 it starts the same stumble but BAD and jerks and what-not. I push the clutch in, and it returns to a 600-650rpm idle with the occasionaly muffled pop from under my ***.

May not be related, but I have to crank it for a good 2-4 seconds before it starts. It starts easier when cold. It cranks very fast (Optima Red Top).

Any ideas?

I need to test the compression again, as when I did the first time, I was a retard and didn't hold the throttle down. It only bumped 30-35psi. With the release button unpressed, it topped 110psi. Same numbers on both rotors, all faces.

Maybe I'll do that right now.

 

possibly could be your TPS

 

Updates:

Holding the gas pedal down, it bumped 40psi every face, both rotors. Without the release valve held open, it pumped up to 125psi. (attached is a pic of the compression gauge that was used)

My spark plugs (leading, BUR7EQ)...wow. In each attached pic, the front rotor is on the left.

I wouldn't think anything is wrong with the engine at all, if not for that little muffled pop, the stumble and the hard starting. (and the look of that front leading plug!)

 

TPS was slightly off. Fixed.

All problems still remain.

 

The pop sounds like an afterburn, which is a signature of a too-rich mixture. This may be related to why it starts better cold. Some things to check:

1) trouble codes
2) correct operation of the water thermo sensor, to and including proper voltage at the connector on the ECU
3) check timing and advance
4) set idle mixture

The bucking during decelleration might be a symptom of improper operation of the AAV (anti-afterburn valve). It is supposed to inject extra air into the intake during decelleration to keep the mixture lean and avoid afterburns. There is a procedure in the FSM to check it, as well as the air pump which is its source of air.

When was the last time the fuel injectors were professionally cleaned?

Check the engine and transmission mounts, also the differential mounts. The front diff mount gets old and the rubber puck fails in shear, and allows the rear end to rotate about the axis of the rear axles; could be related to your bucking.

Set up a monitor lamp (see my posts on the 'green lamp check') and remote it to the cockpit; it should be on (rich) during acceleration, flashing during cruise, and out (lean) during decelleration. If it is on (rich), or flashing (alternately rich-lean) during your bucking, that would implicate the anti-afterburn valve, fuel injection, etc.

Also, if the monitor lamp does not flash during steady cruise operation, this means the ECU is not going into closed loop. That usually means it is not seeing a valid O2 sensor signal, and is running in a default rich mode, which might explain the afterburn/bucking during decelleration. If this is true, it is usually (but not always) accompanied by a trouble code 5 (O2 sensor).

If none of that pans out, I would check for restricted exhaust/blocked cat; remove the O2 sensor and go driving. It provides a relief path for the exhaust. If the problem goes away, suspect an exhaust restriction.

 

Looking again at your pictures of the spark plugs, it appears to me the front rotor primary is fouling and the rear is not. This suggests that something is going on in the front rotor that is not in the back. Some possibilities might include:

1) the front and rear plugs are not really the same, front is too cold a plug. But you say they are both NGK 7s, so that should not be an issue...
2) oil is getting into the front rotor: bad oil rings, or excessive output from the oil injector for the front rotor
3) mixture is too rich on the front rotor, ok on the rear. This could be a result of dirty/malfunctioning fuel injectors, or maybe a vacuum leak in the UIM or LIM area that is providing extra air to the rear rotor only, and the mixture has been adjusted up to compensate...
4) one of the problems outlined in my previous post is causing the motor to run rich, but just on the edge of the 450 degree self-cleaning temperature of the plugs, and the front rotor for some reason is slightly cooler than the rear, enough to put it under the boundary...

 

Thank you Cal!

I'll switch the plugs this evening, to see if the fouling is a result of manufacturer error, as both plugs are labelled BUR7EQ. If the front is still fouling, I'll move on to the rest of the list.

I'll search through FAQ's and what-not to find instructables on the processes and checks you've described.

The O2 sensor is brand new, about 2 weeks old, and the NGK 7's were bought at the same time.

I don't know the last time the injectors were professionally cleaned. Not since I've had the car, about a month.

I've experienced busted/torn mounts before, nothing in the mannerisms of the car feels/sounds like bad mounts anywhere. I'll check anyways.

Looks like a run to RadioCrap soon.

Thanks again!

All other opinions are welcome and appreciated!

 

Could it be the fuel filter or fuel sock (pre-filter at the pump)? Well, after typing this, I guess not if it's symptoms indicate running rich.

Also, any comments/concerns on my compression numbers?

 

I was wondering about the condition of the fuel injectors, based on the different condition of your spark plugs. If one rotor is running richer than the other, i.e. one injector is slightly clogged, flowing less than the other, the closed loop system would run the mixture rich to approximately the average of the two. Thus one rotor would be a bit rich, the other a bit lean. If the rich rotor were under the 450 deg self-cleaning threshold, it would tend to foul while the other would stay clean.

 

That makes sense, the O2 sensor would tell the ECU to spray more fuel, and in doing so it runs the good rotor rich...Hmm...

It just finished a tank that had about 6oz of MMO in it...I dunno how to check the injectors short of taking them out.

 

It will tend to adjust the mixture to a point halfway between the rich and lean rotors; at which point one will be running on the lean side, the other on the rich side...

There is a procedure in the manual for flow checking the injectors on the car, but it is no less work than taking them out altogether. I just finished that job, it is not so bad, just a little tedious. I sent the injectors out to be professionally cleaned ($98 for 4), and am very happy with the results. If it has never been done on your car, you might consider just going ahead and doing it. Supposedly it should be done every 30k miles anyway, and five 'l get you ten yours are way past that... also the cleaning shop will typically test them before and after cleaning and give you test reports. If you mark the injectors, you can then tell if there was a problem and which injector/rotor it was on.

Assuming your compression is equal (front vs. rear), then it is a good bet that the rear primary injector is dirty, restricted, and flowing less than the front primary. At least, that's my bet...

If cleaning does not solve the problem, at least you will have eliminated one possibility.

Does your car exhibit any of the 'dreaded 3800 rpm hesistation'?

 

I'll go through the injectors first chance I get. Thanks!

I don't get the slightest bit of the 3800 hesitation. I actually look for it often. I also went through and cleaned all of my grounds first thing after buying the car...maybe that had something to do with it?

Also to note, my pulsation damper isn't leaking. I've read it's a common and expensive issue.

 

I have battled the 3800 rpm hesitation for years, and just solved it last week with a fuel injector cleaning. I noted, using a monitor lamp remoted to the cockpit, that the car was going lean during the hesistation. My car already had all the gounding mods, to no avail.

After the cleaned fuel injectors were installed, the hesitation disapeared in terms of my ability to feel it; it tachs right through the 3800-5500 range like it was never a problem. I can, though, still see about a 1/2 sec lean spot on the monitor lamp.

I was thinking that if you were having the hesitation that it might be related to a clogged injector (too lean). But based on your last, I think it is more likely that you might have a clogged (and therefore leaky) injector on the front rotor; such might cause that rotor to run rich all the time. Being rich, rather than lean, you would not have the hesistation.

That's my theory....

 

Still havent checked the injectors yet.

Any input on my compression numbers?

 

GOT IT!

I bought a couple more N/A 13B's a little while back. I took the injectors out of them, soaked them in diesel fuel overnight, sprayed them out backwards with the air compressor at 120psi, installed them...car runs great!!!

Now my front diff mount is shot...dammit.

Also, between my last post and now, I had to replace the water pump, that was fun.

How bad is it to drive with the bad diff mount? I mean, I know the noise and thud can get annoying, but am I damaging anything terribly bad? Cause this is my daily driver and I can't have it down for more than a day, so until a new mount comes in from mazdatrix, I have to drive it to/from work.

Thanks guys! (especially Cal)