Okay, this ones driving me nuts. Have a '92 t/a with a 350 tpi and 203,000 miles. Before I rebuilt it a couple weeks ago, it had a substantial hesitation on acceleration from a stop, along with a heavy miss at cruise on the hwy.. We ironed out the miss with a good cleaning/ flow match of the injectors, but the hesitation still exists, only now it occasionally dies when you come to a stop, and the idle is sometimes erratic. The engine also seems to stumble along at about 1400-2000 rpm during low speed cruise, and has a new vibration at that speed as well. I have checked the tps (.54 volts closed, 4.5 or so open), the numbers climb in a linear fashion, with only a minor jump (.02 or so) when you first hit the throttle. I have also tried to adjust the min. idle air setting, but it doesnt work the way it was described to me, in that I can only get a min. speed of 1400 rpm or so with the connector off. The computer is throwing no codes. I will mention the mods for "what if " purposes, though the condition existed before they were done.
-minor plenum porting (baffles eliminated and minor smoothing)
-headers (edelbrock shorties)
-flowmaster cat back setup (car has factory dual cats)
-initial timing set to 12 deg. (yes, I disconnected the wire first))
-underdrive crank pulley
-throttle body heater bypassed
-speed pro hypereutectic pistons (could this be my vibration?)
-converter modified to stall at 2000-2500 rpm
-180 deg. thermostat
-previous owner set the fans for constant on (I dont think this is good....?)
Otherwise, engine is stock. A cold air induction and K&N conical filter are currently under construction, but not yet installed.
If I stick my foot in it, it seems to go just fine, but if I drive like a civilized human, the car falls all over itself until the r's come up.
This is my wife's daily driver, and she REALLY wants it back, but I wont let her have it until its right. Any help anyone can give would be greatly appreciated.

 

My car does the EXACT same thing

I assume its my cam though, i got a mail order tune...im gunna datalog the car and send it back for a more exact tune. hopefully that fixes it. The car has never died on me but it sounds like it does when i come to a stop at a light..it sounds like its just about to stall, but then it comes back. I get no codes either...and it stumbles just a bit when you get on the throttle at first.

As for the vibration at 1400rpm...my car does it and ive determined its the 700R4 slipping. It likes to do it, epsec in overdrive at around 40mph...sometimes when it goes into 3rd at a low speed too.

Im pretty much thinking all the problems are because of the trans...it doesnt downshift right when you slow down and i think thats what causes the car to act like its going to stall.

 

I think its something to do with one of the sensors. But I would start out with doing all the regular tests, run a vacuum test, test the fuel psi at the fuel rail, run a compression/leakdown test, and pull the ECM codes if there are any, or even better hook up a scantool. The reason I think its a sensor is because it runs good when you go WOT but runs bad when you are cruzing. Does it also run good when you first start it, but then gets worse once it warms up? These changes (WOT vs cruze and changes in temp) affect the mode the ECM is in as far as open/closed loop. When the car is below normal operating temp or the throttle is at WOT the ECM will be in open loop meaning its not making changes based on sensor input. When the car reaches normal operating temperature and/or is being driven normal like cruzing speeds the ECM is in closed loop meaning its making changes based on the sensor input. You can verify this by using a paperclip on the ALDL port and putting the car into field service mode (ground A & B while car is already running). The SES light will blink differently depending on what mode the ECM is in (open/closed loop). Might want to try a search for more info on that, dont remember how the lights are supposed to blink. Doing this test won't tell you exactly what the problem is, but it will help you narrow it down. If you tried everything and you still cant find a reason I would blame the ignition system first since its somewhat hard to really test those components. You can do it with a DVM, a search will give you more info, but very time consuming. An easy way to test a portion of the ignition system is to try idling the car with the timing connector disconnected (the one you disconnect to set timing). This will prevent the ECM from changing the timing. So if it acts better with that connector disconnected that could be an indication of a problem with the EST/ESC system, ECM, or distributor itself. Fortunately you can also test those parts with a DVM.

 

Disconnect the EGR, take it for a drive. See if it's improved.

Also, clean the connection at the coolant temperature sensor.
Ohm it out, or use a scantool to see if it's reading steady and accurate while driving.

 

The first thing I would do is take a little timing out - bring it back to 6-8 degree's or so. Clean the IAC as well. The IAC could very well be dirty and can't gain full movement (and certainly not smooth movement). As mentioned above, as over 200k miles, if that EGR is original it may have served it's time.

When put into test mode does the ECM respond with a code 12? Just curious - noplace here had I seen that you checked the ecm for proper function.

 

2 things I'd do first. - 1st is backing the timing down to around 6-8. More than that w/o changes in the chip is just asking for it. - 2nd, start checking for a vaccum leak. If you are properly trying to set minimum air and the lowest you can see is 1400 rpm, the motor is getting air elsewhere...

 

Stealing my timing suggestion......
.... you bastard!

 

some of my best ideas are stolen...errr uh "borrowed"...

 

Hey guys!!!
Off idle stumble was the egr SOLENOID. Now, the car drives much better, and is responding better to other inputs such as timing and IAC and tps adjustments. Still has a little but of a stumble (barely noticeable compared to before...), I will try pulling back the timing a bit. Still has a fairly substantial vibration, I guess I'll have to ask the machine shop about it. I suspect the pistons may have something to do with, as this is a factory external balance engine, and I didnt have the set special balanced. Dunno. Ideas? Thanks for the help on the other stuff though!!!!!!

 

Whoaa..... hold on here...

How far out of balance do you think the rotating assembly is? You are going to end up burning up that motor very quickly if it's too bad. I've seen bearing go in just a few weeks. That should be a immediate concern to you, especially if you think it is so far gone that it's causing the whole car to vibrate!

 

where is the egr selenoid on our cars? is it mounted in the center of the lower intake?...that thing that the vaccum line goes too?

and if so how can i test if its bad?

 

Yes a vac line goes to it, and yes it's mounted in the center of the lower intake. There are a bunch of ways to test them, I just unplug the vac line that runs to the solenoid. If it solves the problem - I replace both the solenoid and the valve, since either one of the two can create an EGR issue.

If it doesn't solve the problem, the egr valve itself may be stuck open by a peice of stray carbon, so I replace the valve and attempt to clean up some of port. If that still doesn't solve the problem - it may not be an EGR problem.

Others will have different, more intelligent, procedures.