I am having a head scratcher. 92 z28 has sparatic problem where at a stop light and sometimes driving, the rpm will swing up and down and sometimes idle up to 1600 rpm at stop light. I have replaced the distributor, checked the ground wires on back of motor and by battery and repaired one bad ground wire. There are no codes in the computer. O2 sensor has been replaced, air idle motor removed and checked, ok and working. Knock sensor scanned fine on operations, checked all over for vacuum leaks and none found (used up 2 cans of carb cleaner looking)...... Unhooked and plugged brake booster line and no difference apparent so rehooked. Carries 38 lbs of fuel pressure. Map sensor appears to be fine and operating. Hard to inspect egr valve but car is not smoking or burning unusual amounts of fuel. Plugged reman ECU in and no change. Installed new fuel filter. Not sure what else to check. Car may run fine for awhile and then, seems to be at stop light more than anything, engine will surge to about 1600 rpm, and then maybe fall on it's face. Sometimes it will stumble when driving and then clear up and run ok for awhile. I am out of ideas but would appreciate only knowledgable replies please. Once again, vehicle scans with no codes to be found.......Thanks for your thoughts, Scott......PS.....not the car in the pics....

 

What kind of fuel induction do you have?

 

topradman
With 38 PSI of FP your fuel injection system is at a lower limit of TPI. Which may explain why it is surging, since a leaky injector may not adequately allow fuel atomization, hence resulting in inconsistent fuel delivery. Check plugs, see which cylinder is different - check injectors.

//RF

 

I have been having the same problem. Does yours only do it in closed loop, if I unplug my O2s forcing it to stay in open loop it runs fine? And I have also check dam near everything just as you.

 

topradman, my guess would be the stock Multec injectors are on the way out. Check the resistance of the injectors, and that are less then 12 ohms are bad. Any that are less then 14 ohms are on the way.

Was the fuel pressure of 38 psi checked with leaving the vacuum line connected and the engine running?

RBob.

 

The car is TPI 5.7L, the 38 lbs was with car running and nothing unhooked (i.e. regulator) it has done this closed & open loop. I will check the resistance and see what I can find. I wlll post results. thanx fellas.....

 

you hit it right on the head. i had the same exact problem on my 91 corvette. the multec injectors are garbage. i had 4 bad injectors and the car only has 54000 miles on it.

 

All my injectors check out at 16 ohms. What's next? Installed new O2 sensor ( it was bad) replaced MAP sensor, no changes. Does this surging stuff when it wants to still. Any other thoughts? The IAM seems to be working properly. TPS seems to be fine. Ground wires seem to be fine. Installed new computer and no changes (returned that to Advanced ), new cap & rotor, I did have a value on my scanner of 0% CCP but not sure what that is. It didn't change with throttle position.

 

Check your throttle return spring.

 

I had a similar problem with my car after my header install. The engine was surging to about 2000 rpms at stop lights then would idle back down. Ended up being the EGR valve plastic hose/line had split in half and wasnt allowing the EGR to work properly, causing the surging. Also happened to my buddys SUV, EGR line had fallen off while 4 wheeling lol, but myself and him did not get any codes from what i can remember. I may be really far off the deep end on this one but you may want to just look into that...Good luck and let me know how you make out with it.

 

Also make sure cruise control cable is not hindering movement. My cruise control cable was sometime holding throttle at 1% rather than allowing it to go back to 0. This was while the vehicle was idling while I gathered data with a scan tool. I readjusted the cable bracket on the plenum which eliminated the problem. I had uninstalled the bracket a week earlier while replacing injectors.

 

EGR is easy to check
use a mityvac to actuate the egr
while cold with you're hand check the movement of the egr valve it should be smooth
make sure it doesn't stick at the bottom(no vac)
Egr will make the engine stall on idol
the surge could be from the ecu trying to compensate by adjusting the BLM
until it falls flat on it's face
go with a good egr valve for replacement
when i rebuilt my Cammy
by accident i had found a bb stuck in the egr vacuum line seems the previous mechanic
had some idea in his head that it would fix it
so new egr
no stall

 

with all the gessing going on. you almost had it.. when you posted your MAP looks ok..

if at any time your car just dies and a will not restart.. for about 15 to 20 min. it's your Map.. (will not get a code)

surge
this is also the first signs of something going wrong with your map.

this comes from working with the 91 to 92 z28's
THIS was a prob with My 92 for a few weeks.
had to just Hook it up and let it run for like Hr till it did it..
then about a year later my 89 S10 did it.. the same thing..
swaped out the Map..it was fixed..a few budys cars had the same thing.
new map and the prob was fixed..just so ya know

 

it seems that he has replaced the MAP

 

check the plug.. and Vac..

 

i have the ssame problem but with my 87 2.8 and i have replaced my maf sensor, tps, and the idle air control valve and it seens not to work. ive had some mech. tell me it could be a intake gasket so i dont know wich way to go????????? i also work at advance so it could be a lil cheaper for me.