My 89 is giving me fits. I recently made it into a 383 and even more recently installed the FIRST Injection systems complete unit consisting of bigger runners, a 835cfm throttle body, intake and plenum. My problem is that it just won't idle properly. It keeps seeking and varies between 500 and 1400 rpms, constantly going up and down and up and down etc. This creates a real problem trying to put it into gear in any direction, as it stalls if I don't feather the brake/gas pedal until I get it rolling. Once over about 1000 rpms, it runs fine. It just won't idle smoothly at a normal steady rpm. I checked for vacuum leaks and found none, and the the TPS is working ok. I'm stumped, as is my tuner. Anybody have this problem or have a cure. I'M GETTING DESPERATE!!!!

Steve

 

Try and make the idle richer, usually a surge is related to a lean condition. If that does not work try killing your low rpm spark advance so the car does not have enough power to very quickly rev up which causes the engine to overshoot faster than the IAC can compensate for. I will lower all spark advance from 1200 rpm to 6 degrees and that will usually stop the surge. The 6 degrees advance does not cause drivability issues because as soon as you give it gas the torque convertor jumps above 1200 RPM and the advance returns to normal. Try playing with the air fuel ratio first and then the spark advance. When you use the spark advance fix it can sometimes cause stalling issues with the car if you start it with the air conditioning on. The added load causes a surge and then the IAC sometimes cannot recover quick enough.

 

FWIW, my 87 is doing the same thing. The tune-up guys at our shop have checked everything with their scan tools and don't see anything wrong. Fuel pressure is good and the IAC is clean and operating. TPS voltage is where it should be. They did recommend cleaning the injectors, but I have been adding a bottle of Techron at each fill-up even with buying Chevron gas.

One thing I have noticed is that once the surging starts, I can turn the A/C on and the surging goes away. Turn the A/C off and the surging comes back.

A vacuum leak would be a common problem with older engines but with your new gaskets, I start by re-torquing all the intake bolts. You could use a propane torch (UNLIT!! ) and point the gas at the intake gaskets to see if the idle increases. A bad fuel filter "may" be a problem too. I have not changed mine so that's on the short list to do.

Isn't 6 degrees BTDC the factory timing setting? The ECM controls spark advance, not the distributor.

 

I agree. A couple of times I've had surging, on both FI and carbed engines, it was a lean condition causing it. Other times it was caused by a vacuum leak.

An adjustable fuel pressure regular could be tried to help richen the mixture. Some poo-poo that suggestion, but it's worked for me in the past. Even raising the TPS voltage to around .67 may help. Nothing ventured, nothing gained.

How 'bout posting more specs on your setup, like cam, ECM programming, etc., just in case it turns out to be something else we should consider.

Lots of guys suffer this but it's fixable.

Jake

 

I mean 6 degrees in the tune. The factory setting is a base point of 6 degrees and then it adds in a programmed amount. At idle it could easily be as high as 20 degrees. When I refer to 6 degrees at idle I am refering to the total advance.

 

There may be more to this than anyone thinks. Whether the A/C is deleted or not......and I don't know what it the answer is....but using common sense and a good guess.......something tells the motor to idle up when the A/C clutch grabs.

What is that something? how and what makes it work?

 

The AC adds in a load which causes the engine to accelerate slower, this slower acceleration allows the IAC to control the RPM without overshooting its target RPM causing the surge.

 

My guess is that the ECM signals the IAC to compensate for the drag of the engaged compressor. The A/C controller has to be tied in to the ECM; some sort of signal is sent when the A/C is running. Would a scan tool show some sort of change in a value when the A/C is engaged?

 

When the AC is engaged the engine RPM is spead up by the RPM commanded in the program, sometimes it is 50 RPM sometimes it is nothing. You need to review the bin file for that car to see what the idle change if any is.

 

Some questions:

Auto or manual?
How big is your cam?
What is your initial spark advance?
What is your desired idle rpm vs coolant temperature?
What is your spark advance in the vicinity of the desired idle rpm?
Are you in open or closed loop when problem occurs?
Injector size? Has this been accounted for in the tune including both offset tables?
Fuel pressure?
Battery voltage at idle?
MAF flow at idle?
IAC position at idle?
tps voltage at idle?
BLM at idle?

As already discussed, the surge is due to a combination of mixture error and unstable spark advance tuning near the desired idle rpm, with several additional potential contributing factors depending upon what you have in the car now.

 

I have decided to post the spark advance tables from a normal non surging engine. This happens to be a 355 superram combination that idled fine without a surge. Notive the spark advance at low RPM and load values.

Main Spark Advance Vs. RPM Vs. Load

RPM Load Value (LV8)
32 48 64 80 96 112 128 144 160 176 192
400 25.0 25.0 25.0 25.0 25.0 17.9 12.0 6.0 6.0 6.0 6.0
600 25.0 25.0 25.0 25.0 25.0 17.9 12.0 12.0 12.0 12.0 12.0
800 25.0 25.0 25.0 25.0 25.0 25.0 22.1 17.9 14.1 14.1 14.1
1000 25.0 25.0 29.9 29.9 29.9 28.1 25.0 20.0 15.1 15.1 15.1
1200 25.0 25.0 34.1 32.0 29.9 29.9 27.1 22.9 16.2 16.2 16.2
1400 25.0 25.0 35.9 33.0 33.0 33.0 28.8 26.0 20.0 17.9 17.9
1600 25.0 38.0 38.0 34.1 34.1 34.1 30.9 28.1 22.9 21.1 21.1
1800 25.0 38.0 38.0 35.9 35.9 35.9 33.0 29.9 25.0 22.9 22.9
2000 29.9 38.0 38.0 38.0 38.0 38.0 38.0 34.1 27.1 25.0 25.0
2200 38.0 38.0 38.0 38.0 38.0 38.0 38.0 34.1 27.1 27.1 27.1
2400 38.0 38.0 38.0 38.0 38.0 38.0 38.0 34.1 27.1 28.8 28.8
2800 38.0 38.0 38.0 38.0 38.0 38.0 38.0 34.1 27.1 28.8 28.8
3200 38.0 38.0 38.0 38.0 38.0 38.0 38.0 34.1 28.1 30.9 30.9
3600 38.0 38.0 38.0 38.0 38.0 38.0 38.0 33.0 28.1 30.9 30.9
4000 38.0 38.0 38.0 38.0 38.0 38.0 38.0 33.0 28.1 30.9 30.9
4400 38.0 38.0 38.0 38.0 38.0 38.0 35.9 32.0 28.1 30.9 30.9
4800 40.1 40.1 40.1 40.1 40.1 40.1 38.0 34.1 30.9 30.9 30.9
I have now posted the spark advance tables from a surging 406 surging that had a superram. Notice the spark advance tables at low RPM, this eliminated the surge.

Main Spark Advance Vs. RPM Vs. Load

RPM Load Value (LV8)
32 48 64 80 96 112 128 144 160 176 192
400 15.1 15.1 15.1 15.1 15.1 15.1 15.1 15.1 20.0 20.0 20.0
600 15.1 15.1 15.1 15.1 15.1 15.1 15.1 15.1 20.0 20.0 20.0
800 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 20.0 20.0 20.0
1000 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 20.0 20.0 20.0
1200 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 20.0 20.0 20.0
1400 20.0 20.0 29.9 33.0 33.0 33.0 28.8 26.0 22.1 21.1 21.1
1600 20.0 20.0 38.0 34.1 34.1 34.1 30.9 28.1 23.9 22.9 22.9
1800 28.8 38.0 38.0 35.9 35.9 35.9 33.0 29.9 25.0 23.9 23.9
2000 29.9 38.0 38.0 38.0 38.0 38.0 38.0 34.1 27.1 25.0 25.0
2200 38.0 38.0 38.0 38.0 38.0 38.0 38.0 34.1 27.1 27.1 27.1
2400 38.0 38.0 38.0 38.0 38.0 38.0 38.0 34.1 27.1 28.8 28.8
2800 38.0 38.0 38.0 38.0 38.0 38.0 38.0 34.1 27.1 28.8 28.8
3200 38.0 38.0 38.0 38.0 38.0 38.0 38.0 34.1 28.1 30.9 30.9
3600 38.0 38.0 38.0 38.0 38.0 38.0 38.0 33.0 28.1 30.9 30.9
4000 38.0 38.0 38.0 38.0 38.0 38.0 38.0 33.0 28.1 30.9 30.9
4400 38.0 38.0 38.0 38.0 38.0 38.0 35.9 32.0 28.1 30.9 30.9
4800 40.1 40.1 40.1 40.1 40.1 40.1 38.0 34.1 30.9 30.9 30.9

Have your tuner read this and see if it makes sense to him.

 

C4cruiser, See I don't know the answer...I'm just stabbing in the dark but bjank is getting to the meat of what it "could" be.

When dealing with computer controlled issues, what input are we providing.

I may be sending this thread into a dark chasm......but I'm trying to ask the "right' questions.

You two have climbed all over that motor with settings and things yet something is still causing a surge.

Mine dies this as I have an A/C leak....when the clutch kicks in and the R134a is low.....the low side gets sucked too low and the low pressure switch cuts the power to the A/C.....this cause the idle to surge and it is annoying as hell.

I know (of course the A/C isn't blowing too cold, that is the first clue) at that time the system needs another can of refrigerant added. This fixes the surging idle immediately.

I'm not suggesting that you are low of refrigerant, I am just saying..."what if"

What if the "thing" that tells your motor to surge when the A/C kicks on is acting up.....could this be an issue?? BJansk says there is an RPM command......I'm starting to connect a dot or two.

Like I said, I may be sending everyone in the wrong direction so please if you know more than I do (not very hard to know more than I do...I'm not really smart) correct me at this time.

 

Dude, how do you know this in your head??

Awesome.

 

According to my wife I have lots of useless information in my head! I happened to have my tuning computer at work today so I just copied and pasted some of the tables.

 

My `87 did that. A new MAF fixed the problem for me.