Hello all, first time posting

I have a 1991 Camaro RS with the 305 TBI and the 5 speed manual. My dad and I recently rebuilt the engine and put everything back together in the engine bay. It is a big project for us and are covering milestones one job a time.

The current problem that has me baffled is a surge when the engine is hot. This symptom is accompanied by a a high hot idle and stalling when the accelerator is suddenly let dropped, there is a gurgling throaty sound from the exhaust when that happens. I have cleaned the IAC valve and the EGR was leak tested by depressing the diaphragm and plugging the nipple. Vacuum leaks were checked for using an unlit propane torch. Timing was set to 4 degrees with ESC disconnected, it idled well there when not surging. The engine idles perfectly when cold.

In a recent test notes were taken to try clue in to the issue:
When cold it Idles at 2000 rpm; 13psi of fuel pressure measured after inline fuel filter; timing is about 24 degrees advance (hard to tell since it went off the marks);
When warm it idles around 1550 rpm; intermittently, it goes into its surging mode where it goes up to about 1750 rpm and falls back down every ~2 seconds. Whenever it surges the timing retards to about 12 degrees.

In the instances where the accelerator is pressed then suddenly released the the timing drops to 4 degrees.


current engine layout

Any help or suggestions will be appreciated. I am open to any ideas as at this point I am stumped on what to try next.

 

Disconnecting the o2 sensor while surging stops the surging. Does this mean it goes back to an open loop type of mode or give the o2 reading a standard value?

 

Welcome Aboard!

Your assumption is reasonable, but instead of the ECM supplying a nominal value for the O² sensor value, it should revert to the fixed constant for injector pulses, ± the scalars for temperature, RPM, TPS, etc.

Does ignition timing also revert to the 4° base timing when the O² sensor input is nulled?

 

Thank you for the reply!

Right now, the engine doesn't run because the piece on the distributor pole that the pickup coil picks up broke when I was disassembling the distributor. It was super rusty anyways. I read online that "jumpy" timing might cause it which seemed like it with the timing light. Waiting on a new distributor.

 

Replaced distributor and it stabilized the timing. The surging persists. Disconnecting the o2 sensor solves the surging but not the stalling when the gas pedal is suddenly let off, and it stays at about 16 to 18 degrees with the o2 sensor disconnected.

 

What cam is in this engine?

 

The original stock roller cam. Although I am not 100% certain, since I didn't explicitly check for that when putting the engine back together. Everything else in/on the engine and in the car is stock except the radio.

 

Checked TPS today. Probed the signal wire and the ground. Key on engine off. At closed throttle 0.55 volts were measured and at fully open throttle 4.35 volts were measured. Which I think is within spec. Will continue to document until solution is found for my own benefit and whoever else will find it useful.

 

The TPS voltage range should be acceptable.

When the IAC was cleaned, was the minimum air position checked? In doing so, was any wear at the throttle shaft noted?

 

How do you check for minimum air position? All I've read about is how to adjust that or the base idle, not check.

 

Just now extended the IAC valve off the throttle body by jumping the terminals under the dash. Took it apart and put back together. Then I disconnected the jumper and started the car. Once it warmed up it the surging was 99% gone and it stopped dying when accelerated and decelerated quickly. When the accelerator pedal is pressed and then depressed, It now drops rpm and hangs above idle for a moment. Without an actual test drive, the only problem with the engine is a high idle now. It is now at idling at ~1700rpm. I have read about drilling out the plug for the idle adjustment but really don't want to do that. Any ideas?

 

You mentioned earlier that the IAC was cleaned. Was it disassembled, cleaned internally, spring adjusted, and lubricated? That's usually what is necessary.

https://www.thirdgen.org/forums/tech...ml#post6479123

It would be good to check the throttle body bores, plates, and shaft for wear, looseness, and binding. Unless something has been bent or severely worn, it usually does not require adjusting the throttle stop screw, but removing the tamper plug is not all that difficult either. If the throttle shaft is as worn as I suspect, it will be easy enough to pop the plug out once the TB is on the bench for bushing installation.

 

Thank you for the link

The IAC was taken fully apart, cleaned, and lubed. It seems to be set at the right depth and since the surging has stopped I assume it is now good. Only the high idle remains. The throttle shaft end play is about 50 thousandths and the side to side play is tiny but perceptible on the end where the TPS should be. The side to side play makes a slight click noise if that is any indication as to the amount of wear.


TPS side of throttle body

Is that to much play?

 

0.050" is pretty sloppy. That may be sufficient to create a binding condition between the throttle plates and bores, and that could explain the high and random idle RPM if the plates do not return to the same position every time.

 

Giving it a second look, it looks like that in and out .050 inch end play is by design. The figure of .050 inches of play is only present when fully open and is a result of the edges of the throttle plates sliding into the bore in which the throttle shaft resides.

The gap that is "by design"

Again I don't have much experience with the tbi systems and started the car hobby only a couple of years ago so I may be wrong. Was the side to side play acceptable though?

 

Used a smoke machine to find smoke coming out of the EGR. Will change as soon as I get it here and will update.

 

That axial play only at WOT is not a real problem. I was under the impression that it was with the throttle closed.