Jess Cooper

Hey everyone, I have been trying to diagnose a problem with my car for the
last several months and it is about to drive me crazy. I was hoping that
someone would be able to help me before the problem drives me to drinking.
I have a 1987 IROC 305/5-speed. It still has the original LB9 305 with
130,000 miles on it. During a typical quarter mile run, the knock sensor
will pick up about 10 counts and the timing usually gets retarded as much as
12 degrees. I have adjusted the fuel pressure as low as 870 mV to as high
as 960 mV with no change in detonation. I have run top engine cleaner
through the engine in case carbon buildup was causing pre-ignition, but
there was no change. I have a new coil, ignition module, pickup coil, plug
wires, knock sensor and the fuel injectors have been cleaned. The base
timing is dead on at 6 degrees BTDC. I have adjusted it down to 4
degrees...but my e.t.s were worse and there was still some detonation
(timing was retarded about 10 degrees instead of 12). Using a higher octane
gas usually helps a little...but there is still a considerable amount of
detonation. I also noticed that the knock sensor picks up a huge amount of
noise (30-50 counts) if I rev the engine while it is in neutral. I cannot
hear any noise and I have checked the engine over several times to see if
there is a loose bracket hitting against the block. Is this normal?
According to Diacom, my EGR valve is open all of the time (the engine has a
new EGR solenoid, valve and diagnostic switch). But, my idle is smooth and
looking at the EGR valve with a mirror, I can see the diaphragm move like it
is supposed to. Does anyone know how the diagnostic switch works? Does it
ground itself when EGR is detected, or should it be grounded all the time,
but when EGR is detected, it "un-grounds"?

A couple of weeks ago, I disconnected my knock sensor and replaced it with a
3.9k ohm resistor to make sure it isn't picking up a false knock. With the
knock sensor disconnected, I could definitely hear the detonation.

As I was looking over some of my Diacom files last night I noticed that my
Total Spark Advance Relative to TDC was as high as 40 degrees on some runs
and it spiked to 50 degrees on one run. Does this sound right? What value
is good for total advance? Shouldn't it be closer to 30 degrees? The other
question I have is about Diacom itself. In the manual it said that in ALDL
mode, that the spark timing is advanced. Can anyone verify this? I always
use ALDL mode, just because there is more information. Does anyone have any
Diacom files of a properly running engine they could e-mail to me and I
could compare to what I have? Sorry this is so long. Thanks so much for
your help. If anyone wants to e-mail me about this, please e-mail me at
cooperj@sporlan.com

innocence faded

Did you get response to this? I am having similar detonation problems with my car.

JAMES POLEDNA

sound as if you have a normal build-up of carbon in your combustion chamber.....

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bought a wrecked(head on,bad)'84 Z28 rebuilt it in'92 parked it in
'93 and ordered a '93 Z28. Got tired of the 4th gen in '99 and sold it and begin swapping in '85 TPI converted to speed density. engine is all stock except for comp cam. body has '86 IROC nose peice,ground fx and IROC wheels/goodyear 245/50's. IMO this car looks and feels more "racey" than my '93 did. and I love driving it. BTW fuel injection is great I'll never go back to carbs.

steve8586iroc

Have you ran you car in the diagnostic mode and or with out the diacom hooked up to see if there was any gains in et or mph. With the computer in diagnostic mode like the manual said you are adding advance to your timing when you don't need it. So the ecm is having to pull out more advance than it should. The only other things to check would be maby a lifter ticking, bearing knock, piston slap. It sounds as if your car runs well otherwise so I doubt it would be an enternal engine noise. I'm sure others will be able to shed so light on this as well.

Steve

sparks383iroc

I'm having a similar problem. I get up to 120 knock counts in a 5 - 10 min run. Have seen the count jump from 20s to 100+ in 2 seconds.

Q: Could this be caused by my exhaust vibrating against the underbody?

The Dart Pro 1 heads have raised exhaust ports (1/4-1/2 inch) that puts my exhaust real close to the body. I notice that I also pickup some counts when I go over a curb(like a driveway). Could the exhaust be my problem?

Sparks a flyin'.

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1989 L98 IROCZ, Bored .030 over,400 Crank,Dart PRO 1 Heads 200cc intake,ZZ3/ZZ4 Cam,Crane 1.6 RR,Edelbrock 3860 Hiflo TPI Manifold, AS&M LTRs, Edelbrock 58MM TB, Ported Plenum, TCI Reverse Pat Man Valve Body, SVO 24# injectors,TCI Streetfight Stall Converter, 3.23 Gears on Auburn Pro Posi, Some Suspension Mods

Jess Cooper

Thanks for the replies. I have not tried to see if there is a difference in performance between using Diacom in ALDL mode and Diagnostic mode. I will try that the next time I go to the track. I did see my knock counts jump a considerably amount one time. Actually, the car was in nuetral and I reved the engine. The knock counts went crazy. That leads me to believe that something is hitting the side of the block...but I have not found anything. I am going to run some more cleaner through my engine and see if I can break some carbon deposits up. Thanks for your help!!!

Dragroc

Make sure that the timing is set where it is supposed to be...for a 305tpi it is 6*BTDC. If that doesn't help, try getting some 112 octane race gas(leaded or unleaded). Dump 2.5 gallons of that into a your tank and fill the rest up with 93 octane...the mixture will take it to around 100 octane. This is what I have to do to race my car. It works just fine and the leaded gas will not hurt as long as you mix it like I said. After a while you might have to change your O2 sensor...but I have ran more than 30 gallons total of 112 leaded and haven't had a problem yet. Hope this helps.

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1989 Chevrolet IROC-Z28 TPI ZZ4 with TH700R4 and 2.77:1 rearend gears:
-Gutted Air Boxes, Throttlebody coolant bypass, Hypertech Air Foil, Accell Superram Base Intake, Crane AFPR, AC delete, Smop pump delete, Hooker 1 5/8 Headers, Holley Ignition Module, Hypertech Custom PROM, B&M Megashifter...I know there is more, I just can't think of it right now!