Jim85IROC

A while back, I was having problems with a part throttle stumble that I absolutely could not figure out. I changed everything on the electrical side of the motor (cap, plugs, wires, ignition module, coil, blah blah blah) to no avail. Fuel pressure is steady. I determined that my injectors must be shot.

So I put in 8 new injectors from Marren Fuel Injection. Within a few days, the car was running even worse and my scan tool indicated that the car was running very lean. I bumped my fuel pressure to about 50lbs, which is when the real problems started. That's when I got an awful stumble at all RPMs (including idle), and could hear an awful lot of gas "popping" in the exhaust as it burned off. I assumed that some of the injectors (at least on the non O2 sensor side) were locking open because of the high pressure. I yanked those injectors and put in some 24lb LT1 injectors, and now the car is significantly improved, though not perfect. Most of my block learn numbers are in the 118-122 range, with one cell (cell 4) at 108. I need to back off a bit more on my fuel pressure, but that's not the point. Although the car is running significantly better, it's still not perfect. I can feel a slight studder as I cruise, and I can hear small amounts of unburnt fuel burning off in the exhaust. This seems to be while crusing or under light acceleration. Stomp on it and everything's great. I can't seem to hear any exhaust leaks that could be letting oxygen get into the exhaust, so I'm not sure what's causing this "popping". I'm also assuming that this popping is causing a reversion wave which is causing my stumble, but maybe something else is causing both effects?

Any wisdom from the peanut gallery?

trackman44

Disconnect the coolant temp, MAF, and TPS one at a time and see if you get an improovment. One of these sensors is probably not sending the correct signal to the ECM ( not enough to trigger a SES code). Also the O2 sensor could be suspect.

mseven

what ecm is in the car? if it is still maf, the grams p/sec. shown when in cell 4 is to rich, or adjusting the inj. constant would help. However 118 is still pretty rich. mo, it needs more tuning and I would back of the fp to approx.45 MO

Jim85IROC

I've monitored all sensors with my scan tool, and they're all performing properly. Coolant temp is accurate, startup temp is accurate, and the TPS works smoothly. I also swapped the TPS for a known good one at one point with no change. The car is a 91, so I don't have an MAF, but my MAP sensor has been replaced with no change. O2 sensor is functioning normally, and my crosscounts seem to increase at a normal rate.

SunCr

Air pump. Check routing, checkvalves and that air is diverted to the atmosphere upon decel; to the CAT after closed loop.

Jim85IROC

How do I diagnose the air pump and the valves? I know how to remove & reinstall, but I've never done troubleshooting on one before.

Thanks.

SunCr

Start by clamping off the hoses to the Headers while observing Block Learn. If it changes or the problem goes away, the Pump may be directing air to them when it should be flowing to the CAT. Verify by removing the hoses - shouldn't be any air flowing through them once in Closed Loop and whatever your Year uses for a switching device is bad (usually a solenoid or solenoids) or the harness - ground side from the ECM - is shorted to ground. If no air flows but the problem goes away with the hoses clamped off, could be a bad checkvalve. On some, you can see inside with the hose removed and find the flapper broken. Otherwise remove it (helps if the headers are hot) and make sure that air only flows in the direction of the headers.

On decel, air is diverted overboard to keep any unburnt fuel from lighting off. Some years accomplish this with a vacuum connection - others through the solenoids only. Simply disconnect the hose leading to the CAT to see if it goes away.

Jim85IROC

Thanks. I'll see if I can get time this weekend to try it out.

Hopefully that's all it is. Then I can toss it right into the trash. Maybe it'll wind up in the same landfill as the camaro's air equipment.

SunCr

Not always a great idea to trash it - air to the headers at start up gets it to Closed Loop sooner. If you do, consider a heated O2.

trackman44

You did state that the scan tool read that it was running lean but in fact it is running rich. If you have an exhaust leak upstream of the O2 sensor more oxygen is introduced in the exhaust gases making the ECM think there is a lean condition. The ECM in turn will turn the injectors on longer thus introducing more fuel into the engine when it doesn't need it. Also O2 sensors are known to give off incorrect voltages to the ECM. So it wouldn't hurt to replace with a known good one to see if things improve.

Jim85IROC

With the current injectors the scan tool indicates that it's running rich. With the old injectors that I bought from Marren, the scan tool indicated a lean condition and the car barely ran. I think the situation there was that a couple injectors were locking open due to the fuel pressure, but were likely on the passenger side where the 02 sensor wouldn't know it.

I disconnected and plugged the air tubes coming out of the diverter and drove it to work today. The popping and stuttering seemed to be greatly reduced, but still present in a very minor way. I backed off a hair more on the fuel pressure at lunch and took it for a short drive and it felt good. Tonight after work I'll monitor it with the scan tool while driving home.