Basically stock 87 L98 with head/plenum/runner/intake & exhaust manifolds porting, MAF screens removed, throttle body water bypassed, and stock Z-52 exhaust system & mufflers. Timing set to stock setting. Injectors recently replaced with new Bosch units. 96K miles, but engine is in great shape with leak down values in the 92% range. Valves were touched up recently when I replaced a failed head gasket. Fuel pump is the original, with adjustable regulator.


Have noticed for quite some time (both before and after the injectors & head gaskets were replaced) that the engine screamed when not fully warmed up, but lacks full power when up to temp (have used both 160 and 180 thermostats with no effective change). This difference is much more than just cold inlet air vs. warm. It would appear that it is more a function of when it goes into closed loop, that performance drops off (even though in closed loop, I think that it goes strictly by the computer map when in full throttle - the oxy. sensor is removed from the loop in full power). The engine does not miss, and no codes whatsoever. Just lack of power in full throttle when fully up to temp. This would lead me to think it is a air/fuel ratio (or computer map) problem. I guess some of the lack of power could be that we are stuck with oxygenated fuel here, but that does not explain the cold vs. warm diff. Fuel economy seems less recently, but not by a huge diff.
I have measured full pressure at the fuel rail (around 40psi) during a run, so pump/filter are not the problem.

Any ideas?

Thanks,
Plasticman

 

If fuel pressure is 40psi at WOT (or with vacuum line disconnected) you need to bump fuel pressure; shoot for around 46psi with vac line off.

Try a K&N filter with cut lid or install the SLP ram/cold air setup. This will get denser air in and support more power.

You might also try bumping base timing a few degrees for crisper throttle response. Also check the TPS values at base idle (450rpm) and at WOT, with engine off of course.

 

Interesting question.

I thought this was probably normal as both my 89 L98 and my LT4 always acted this same way....best performance before fully warmed.

 

You need to ask this question in the Scan and Tune section.

I believe some of the computers tables have coolant temp vs. spark advance (spark advance can make a big seat of the pants difference if its not optimized).

I am assuming as coolant temp goes up your spark advance goes down...

Some of it to may likely be the fact that you are sucking cooler air, but I dont think all of it is due to that.

 

My LT-4 does the same thing.

Larry
code5coupe
:seeya

 

By chance is it an automatic???

I've noticed this type of behavior in older cars with ages A4 transmissions... At first I thought it was sluggish performance on the engine's part, but later started noticing the shifts weren't as firm when warmed up, downshifts weren't as fast, etc...

I put an aux trans cooler on the car run inline with the radiator unit and it helped...

Of course, if you have a manual trans car this obvoiusly doesn't help...

 

65ZO1,

I will check these out (when Spring comes - it's in hibernation right now). I previously tried fuel pressure ranging from 36 on up to 44psi (diff. runs on same night), and found that 40 seemed like the best at that time. That has been a few years ago, and fuel has changed in this area, so maybe pressure should as well (to compensate). Good idea, and thanks. Besides, I did change the injectors (were Lucas, now Bosch) as well, and should of looked at the pressure needing to be changed when I did that.

I tried low restriction air filters (Diacom tested runs with 3 variations, including no housing), and no diff. at all (all run the same night, one right after another). Kind of proved to me that K&N's are a product being sold on hype. The previous owner of my 93 GMC Typhoon had one installed, which I later changed to a stocker type paper filter ( changed it after finding it difficult to clean & oil the K&N - wife won't let me use her "parts" washer) - same result, no diff. in performance.

LT4BUD & Rocco16,

I hope it is not normal, but maybe it is. I have too many other toys, and have neglected the 87 as a "daily driver" until recently.

DIZWIZ24,

I have also asked the Scan/Tune folks per your suggestion! Thanks!


Wake,

It is an Auto, but the shifts are firm and crisp (installed a B&M kit a while ago that did wonders - not just the shift plate kit, but the full Super Trans Pak). This seems to be all in the engine.

The Scan/Tune folks have lead me to a new tack. I will break out the Diacom this Spring, and make some runs (when not fully warm, and then fully warmed up), and compare them directly (as well as maximize the performance with the adjustable Fuel Pressure Regulator, and then some runs with various timing changes). Give me something else to do.

I was just wondering/hoping if there was a "common fault" and if there was a "quick fix". Doesn't seem like it!

Thanks all,
Plasticman


[Modified by Plasticman, 11:48 PM 2/15/2004]

 

Damn I have this in my 86 - but thought it was just my imagination :(

It just seems to bog down a bit once its really warm - compared to when its still coolish - just a touch of the throttle and it feels real responsive. I was thinking low temp t-stat - but if u`ve tried that then i guess its one thing less to think about.

I guess u`ve checked the obvious like brakes binding when warm etc. - i had this once on another car - but its not the problem on this one......

Good luck,

Dom

 

I have the same problem. I have a 1990 corvette coupe with the L98 engine. The first 10 minutes or so that the engine is running it makes awesome horsepower. Stomp on it and it will break the rear tires loose in 1st, 2nd, and 3rd. Then once it warms up, it just loses all kinds of power. It loses that killer horsepower in a hurry too so your assessment of the computer causing it makes sense. On a cold day where the temp never gets above 160 degrees still doesn't make a difference so I don't think it's the actual temperature of the engine that is limiting it but rather when it senses it is warm enough something is happening. It almost feels like someone is putting their hand over the air intake and choking off some of the air. If you ever figure out what is doing it please let me know alzado77@***.net. If I figure anything out I'll let you know. Something definitely isn't right. It is too much of a drop off to be normal. I mean it's still fast, but no where near what it should be. Thanks for the post, it's good to see I'm not alone cause it's been driving me crazy. Al

 

My 87 has always been like that and it's even more noticeable since the mods. I hope this is something that can be ironed out w/ ECM tuning b/c I can put the TPS, timing, idle set and fuel pressure settings any which way but once my oil temps hit 180 power drops big time.