do certian cylinders run leaner?

dthompson

Registered User
Ive got a question for the experts about individual cylinder temps... Do certain cylinders tend to run leaner/hotter than others? I have searched on the topic, but did not find anything.

I have noticed that the primary #5 is hotter than the others. #1 is warmer too, but not as warm as #5. I was wondering if the intake manifold design led to some cylinders taking more air than the others. At first I thought this was a vacuum leak problem, but I have sprayed carb cleaner all over with no change. Also it seems to be these certain cylinders.

Ive recently had the car tuned, and the overall AFR is really good. The shop suggested that i get the injectors flow tested to see how well they match. I would like to know more about what people have experienced in this area.
 
3 and 6, respectively, run leanest. 1 and 2 run richest. How much hotter are they and under what conditions? What do the plugs look like?
 
im still trying to locate a temp gun to read each primary, but #5 starts to glow. Its not glowing cherry red, but you can tell it is hotter. It starts turning red where the kooks header bends down right after it leaves the head. #1 is just barely changing color to glow. It doesnt seem to follow any one situation. I have seen it at idle after driving around and after a pull. i have not seen it get bright hot cherry red yet in any condition.

I will pull a couple plugs today if i get a chance. The honey do list is in effect today, so hopefully i can get a picture of it for you.
 
Vacuum leak is first thing that comes to mind. Does it have a rougher idle after installing the FMIC?
 
i m guessing they are a little on the lean side. If there is a vacuum leak, i cant find it. I have sprayed carb cleaner at every joint i can find. I dont hear a leak either. The gauge still shows about 13 in of vacuum which is what i had before (218/224 cam)/. The car ran ok after the IC was installed, but it wanted to cut off after coming down from a rev about half the time. Ive had a slight problem with the rpms dropping to far to fast since my first tune, but we worked a lot on that yesterday. It is much better now in that department. looking at the dyno printout, the AFR was around 12.0 the entire pull. (when they were doing the pull i was watching it and i thought it was closer to 11.5) There was no knock at all yesterday.
 

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You won't find a vacuum leak using carb cleaner unless it's extremely bad. The EEC compensates too fast for you to notice unless you are datalogging while doing it.

That the motor changed characteristics after installing the FMIC indicates a change in system sealing because the FMIC by itself should make no difference at all.

Are you sure that the injector wires are correct? It is possible and very common to mix up 1-2 injector plugs and 5-6 since they are close together. When new they were labeled, but by now often the labels are gone.

1 & 5 are also on the same coil so there could be a connection there.

Plugs look normal.
 
i did not remove the injector wiring or change the coil. its the same set up as before.

When i did my FMIC, i removed the supercharger to make it easier to replace the return plenum (i had the flanges machined off to make a nice fit for the silicone coupling. I installed the return plenum using the gasket and copper rtv on both sides, which is probably stupid to seal it twice like that. while i had the sc off, i noticed some coolant (small leak) where the lower intake manifold meets the block. i assume it was coming from where the intake meets the heads. It was not coming from the thermostat housing neck thing. I found that two of the bolts near the front of the passenger side were a little loose so i tightened them up and checked all of them. I also installed a catch can to help prevent oil from getting into the intake. I used copper rtv where appropriate on the catch can and good fuel line and hose clamps. i ran once hose from the PCV valve to the catch can and a return line back to were the PCV previously connected.

I dont have anyway to datalog that I know of... my tune is on a SCT chip, not a qh. Ive got some good scan tools capable of reading data streams, but i have not been able to see the data stream on either of the tools with this car.

The other weird thing is that before, my plugs always looked wet. this is the whitest that I have ever seen them since i began this project. before there was never this much white showing, mostly brown on the insulator, and they generally looked wet. This has really got me confused.
 
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Seems like you said you got a handle on it, but trouble catching an idle when off throttle would maybe be iAC, right?
 
so here is a question about something i dont understand. if i had a vacuum leak, and the computer can correct for it, and it is correcting for it...why would the headers glow if the computer is correcting for the lean condition? my understanding is that a vac leak would make the o2 sensors show lean, so the fuel trim adjust to correct the lean condition. I would think if it did all that then the header would not glow. I probably dont have that exactly right, but that is how my brain is processing this problem.
 
i did replace the idle air control valve before the tuning session, but it seemed to make little difference. the original seemed to work as well as the new one.
 
i did replace the idle air control valve before the tuning session, but it seemed to make little difference. the original seemed to work as well as the new one.

People seem to blame the IAC for all sorts of ails, but it's rarely the IAC. When an IAC is bad, the car generally either idles super high, makes rude honking noises, or won't idle at all. When your car fails to catch idle on decel, the problem is that your tune is not matching the hardware. Idle control strategy is the most complicated part of the entire EEC program and the easiest to screw up. Without datalogging capability it's almost impossible to get it right. You can get it to 'work' through throttle body adjustments, but you can't get it 'right'.

As for the EEC adjusting when testing for a vacuum leak, you have to understand what is controlling idle speed. The EEC uses ignition timing as the primary control device for idle speed. The IAC merely sets the baseline but is not a very active player in idle speed control. Instead the EEC has the ability to adjust timing all the way from 0 to 40 deg instantly (yes from one ignition event to the next) to keep the engine on target idle speed. If you've played with a distributor on a running engine before, you may know how much rpm 'control' can be exercised just be turning the distributor. We don't have a distributor, but the EEC is very fast and has a wide range of adjustment available at it's disposal. So when you add fuel expecting rpm to go up, the EEC simply pulls timing out faster than you can add fuel. You have to add an awful lot of fuel through a very big vacuum leak to get it to respond audibly to your efforts.

You also have to remember that the EEC can only see the average AFR (via the 02 sensors) on each engine bank. So if you have a leak on one cylinder causing it to run lean, the EEC will add fuel to that bank until the 02 sensor reads correctly, but only on an average. So you have very well have one cylinder running very lean while the others run rich.

Because your plugs all look pretty even, I don't think you have a problem with one dead cylinder, but instead I think the glowing headers is probably caused by retarded ignition timing. Why you have retarded ignition timing is the question. A vacuum leak is still a very good possibility.
 
I dont have anyway to datalog that I know of... my tune is on a SCT chip, not a qh. Ive got some good scan tools capable of reading data streams, but i have not been able to see the data stream on either of the tools with this car.
.

Had an exhaust sniffer on it lately?
 
As for the EEC adjusting when testing for a vacuum leak, you have to understand what is controlling idle speed. The EEC uses ignition timing as the primary control device for idle speed. The IAC merely sets the baseline but is not a very active player in idle speed control. Instead the EEC has the ability to adjust timing all the way from 0 to 40 deg instantly (yes from one ignition event to the next) to keep the engine on target idle speed. If you've played with a distributor on a running engine before, you may know how much rpm 'control' can be exercised just be turning the distributor. We don't have a distributor, but the EEC is very fast and has a wide range of adjustment available at it's disposal. So when you add fuel expecting rpm to go up, the EEC simply pulls timing out faster than you can add fuel. You have to add an awful lot of fuel through a very big vacuum leak to get it to respond audibly to your efforts.

Dam Dave,
that's the best explaination I've heard for 2 things I never had a strong grasp on. :p Why the IAC is so rarely a problem, and why trying to find a vac leak with out a smoke test IS :D
Thanks
Adam
 
KMT, i had an exhaust sniffer on it friday during the tuning.

What is the best method for doing the smoke test?

I have tried propane, carb cleaner, and I even did a smoke test to try find a possible leak. My smoke test did not work very well. When i did the smoke test i rolled up a piece of paper lit it an blew the flame out and it made a pretty good stream of smoke for a while. What i observed was the smoke going straight up off the end of the paper roll and it was really hard to make it go where i wanted it to go. how do you make the smoke go where you want it to, instead of straight up off the end of the "smoke generator"? i almost think i remember someone talking about a cigar. Do you blow the smoke in the area you are looking at. A little help here would be appreciated, I will certainly give it a shot.

Can you data log anything without a QH? is there any way to see the O2s, timing, etc...? If not, is there any way to read the timing using a timing light or is that a stupid question because you dont know what the timing should be at idle and what timing is actually being pulled?

I appreciate the help, knowledge, and experience that you are sharing Dave.
 
A smoke test requires a smoke generator. Some here have made one out of a party smoke generating machine. Most mechanical shops will have one. They generate smoke which is introduced into the motor via a vacuum port and then you wait for a minute or so while the entire engine and intake/IC system fill with smoke. If there is any leaks anywhere the smoke will start to come out from the area. There is no way you can do this with fire smoke. You need an artificial smoke generator.

It is difficult to retrieve any data from an 89 SC without a QH. Since your SC is relatively modded, I think this is one investment you should seriously consider making. Your SCT tune can be converted over and installed on the QH so you don't have to lose anything and if your tuner is at all accommodating he can tune for you with the QH. The software is not that much different than SCT. The investment will be about $450 by the time you are all set up with your tune on the new QH. Just something to consider.

You can put a timing light on and check timing. It won't be a steady number because the EEC constantly varies it, but you can get a general idea. Normal idle spark will be in the 16-26 range. If it is spending time down in the 0-10 range then you need to adjust your TB down (as you lower base throttle setting, timing will increase and vice versa. This only applies to closed throttle idle though. Once you crack open the throttle then timing is fed directly off the tables and will respond much like a normal distributor based timing curve would. Light load and raised rpm will result in about 35-40 deg timing, adding load will cause timing to drop off. When you crack the throttle at say 2000rpm, timing will drop down momentarily to about 12-15 deg.
 
There was a thread here by a member who made a simple smoke tester with a 1 liter bottle, a bike innertube Schrader valve, and a "smoke bomb" HVAC tester. I believe there r several youtube vids of people making them with coffee can, babyoil and cheep soldering irons. Been thinking of making one this way myself.

Adam
 
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