377Z

iTrader: (1)

When I only had my Z28 (1991 LB9/auto totally stock), I noticed that when it was cold out (like, say, ~55F or colder), it was really responsive and fun (for a peanut cammed 305). When it was warm out, especially 75F+, it would feel like something was wrong with it--much less responsive, with very noticeable flat spots in power. When temp was between ~55-75F I would notice premium gas made a nice difference in power (and mileage). The 1991 T/A I recently (totally stock as well) got does the same thing. It used to run REAL doggy when it was warm out before I did the TB bypass mod. This past weekend I was passing by a local gas station that has 100 octane gas, so I decided to get some for the heck of it. Lo and behold, it runs better after the fillup. Awful peculiar for an engine that should officially run on 87 octane. Timing is set at the recommended 10 BTDC. I plan to back the base setting off a few degrees and see if it improves, but I don't expect much.

What gives? Are the rumors true about 305's being really knock prone? Has anyone had this problem and how did you resolve it?

Steve89GTA

I have ABSOLUTELY had this problem. My car morphs into a Civic when she gets hot. Some of my first mods were 160 thermostat, cold air intake, jet fan switch, new water pump, new rad, all in the name of keeping it cool. It helped big time but once I sit in traffic or something, the power loss is there.

I'm gonna try relocating the IAT sensor on the bottom of the TPI which is known to get heat soaked and give false readings. My knock sensor is also original in the car. I'm ready with WinALDL and 2 cables but I haven't got around the monitoring the knock counts yet...

88TAJeff

Don't worry Steve, won't be long now till nothing in your car gets heat soaked.... Winter's coming!

RBob

iTrader: (1)

Yes, it is knock which the ECM is then pulling the timing. I've noticed the same with a 3.1l f-body. Spec'd fuel is 87. In hot weather it got doggy. Went to 89 octane and the power came back (what little there is).

I would set the timing back to 6 deg and decarbon the engine. Can use ATF in oil/gas, GM top engine cleaner, and/or a spray of water into the TB.

RBob.

377Z

iTrader: (1)

I WISH I had this stuff--is it expensive? Where do I look? I'd love to have a handle on this stuff as it would undoubtedly answer alot of questions for me (provided I can learn how to use it )

I'm pretty darn sure the computer is pulling out timing because it thinks its knocking , the question is--is it really knocking and why? I'll definitely try pulling back some base timing first, though I think the hotter days are dwindling here in Illinois.

Steve89GTA

WinALDL is free. I made 2 cables from serial cables and other small parts you can get from an electronics store.

88TPI406GTA

The temperature difference you feel isn't just limited to NA engines...in fact, with smaller turbo engines the difference is really noticable too...my friend with a brand new WRX STI says it feels like he loses 75 HP on hot days...I also feel the difference in my suburban with a 5.7 TBI...

Why? Because the hotter air isn't as dense so there is less power potential in hot weather than colder (90 versus 40 degrees).

It is amazing how big a difference this makes...

377Z

iTrader: (1)

It's definitely not strictly that--if it was, higher octane gas would not show the improved performance, and power would be down across the board uniformly, not like the HUGE flat spots in power like I feel now. When its hot out it feels a great degree better at part throttle under ~3500 RPM since I installed a 180 deg. termostat and did the TB bypass, but it just lays down at full throttle/higher RPM. Plus, relative to every other type of car I have had, I have never experienced this much difference/weird behavior when it was hot out.

You can't argue with physics and what you are saying is definitely true, but I know its deeper then that. I don't have the time now, but would be interested in calculating the thoeretical difference in potential power at different temperatures at some point just to satisfy my curiousity, however.

I think this just may motivate me to step up and get into proper ECM tuning--its off to searching the PROM/EFI boards for me.

88TPI406GTA

You may have other issues going on here...like carbon buildup...that may be why higher octane gas makes such a difference...

I know that on a prior vehicle (1984 Blazer 350) that burned a lot of oil...the carbon buildup was such that by running 92 octane, it made a difference in running temp and performance...

Could you have a similar issue here? How much oil consumption and or how much mileage do you have?

Also...what about the state of tune on your engine? In hotter weather, it may be showing a partial breakdown in your igntion system...

It may be multiple small issues, rather than one large one...tough to tell...

377Z

iTrader: (1)

Motor does not use more than a quart every 3-4000, sometimes less, and has 75K. For all I know, it could have deposits from running like this all this time if the last owner experienced it as well (it used to be an automatic, and at part throttle its much less obvious).

None of the ignition stuff is new (plugs, wires, cap/rotor, etc) but looks good--of course I know I shouldn't go by looks alone. I haven't checked the resistance of the wires, but cap & rotor looked fairly clean inside, and the plugs have some sight tan/white deposits.

I'll bet you're right about this. I'm leaning towards possible carbon buildup as well as wacky GM programming (from what I have been reading they erred well on the conservative side with timing retard). Now I just have to find the time to get to this project (currently changing the clutch in my beater car between night school & full time job).

88TPI406GTA

I'll bet you're right about this. I'm leaning towards possible carbon buildup as well as wacky GM programming (from what I have been reading they erred well on the conservative side with timing retard). [/B][/QUOTE]

I agree...it probably is a combo of both...try an upper intake cleaner...like Seafoam through the tank and through a vacuum line.