TomH91

There have been a bunch of L98 postings lately regarding sluggish performance. I paid close attention because my 91 with 94k is pretty weak - won't spin the tires from a stop, bogs at partial throttle, etc. On the plus side, fuel economy is still terrific. I have changed plugs, wires, distributor cap/rotor, and fuel filter. I serviced the trans and have had it checked since. The engine sets no codes. Fuel pressure is good. I was concerned about cats and precats, but there is good intake vacuum hot/cold and there are no hot spots anywhere on exhaust. Timing was 2 degrees, so I had it set to 6 as specified. The throttle valves were out of adjustment and have been reset to spec. I am nearly out of ideas. The mechanic I resorted to wants to spend some time checking voltage at the distributor to make sure the coil is okay and the engine is getting reliable spark. I guess the injectors could be tired, but she starts easily and there are no leaks. I think I have hit most of the items covered in recent posts. Any suggestions are welcome - have I overlooked something? - Tom

Marcho Polo

my car runs like a champ 99% of the time, but occasionally it'll bog or not feel strong when I romp on it at a stop sign.


I guess you can't do this, huh?:

http://temp.corvetteforum.net/c4/mar...nalburnout.wmv

I don't know what to check out. Seems if your timing was 2 degrees that would have accounted for quite a bit of it. Now that it's back at 6 it should be running good. I wonder how it got to 2 degrees? Timing chain stretched and now it has some slack?

Hope you find out what's causing the problem.




[Modified by Marcho Polo, 4:12 PM 10/25/2001]

65Z01

Are you willing to do some "simple" upgrades or do you just want the most from bone stock?

TomH91

For now, I want the most out of the stock setup because I autocross the car and would like to stay in NCCC Group I (which allows very few changes). I want to improve my skills for a bit longer, and it's more affordable for me to stay stock as well. I am anxious to resolve the performance issue because it is making the car difficult to drive smoothly. Once in a great while the car runs really strong, and I get the car to break the tires loose from a stop - usually when it's cold. Otherwise, no dice. Maybe an ECM issue?


[Modified by TomH91, 5:23 PM 10/25/2001]


[Modified by TomH91, 5:31 PM 10/25/2001]

65Z01

Lower temp and more air flow will do the trick.

Increase the TPS base setting to about 0.65Vdc (above 0.67Vdc may set a code) to improve throttle response.
Get a scanner and increase base timing until you just get a knock count or two at WOT.
Cut back the spark plug ground electrodes.
Clean the TB, port the plenum and upper runners.
Relocate the MAT into the side of the air filter lid.

If allowed, switch to a 160 deg stat and fan sw and TB coolant bypass.

Install a K&N filter and open the air box, if permitted.

If any exhaust work is possible, install a 2 1/2" front Y, hi-flow cat and hi-flow cat-back.

Do all of the above that are allowed within your desired class.

HWoods

94k and you are autocrossing? I suggest installing a new timing chain. I am sure the original one is stretched. Use a good aftermarket hipo, double roller type. ( I have seen stock crank dampers separate at the rubber and you will bet very inaccurate timing) Will your auto cross class allow for an upgraded ignition module, the one in the distributor? Holley, Accel, and Performance Distributors make one. An aftermarket coil may be a good add on if it is allowed or a new stock coil if that is required. The car is 10 years old


[Modified by HWoods, 5:20 PM 10/25/2001]

pablocruise

I'm curious about your cylinder pressures. How 'bout a compression check? Remote, but maybe they're alittle unbalanced or down slightly? Plugs gapped to correct value? Cats might have some slight clogging by now. Maybe loosen the Y-pipe from the exhaust manifolds to just run a few minutes on the freeway and get an impression (it will be loud)?

grumpyvette

--------------------------------------------------------------------------------
I was recently asked if those throttle body flow straiteners(air foils) will give you the 20 hp they claim for that one modification on his 1985 vette , heres what I told the guy;and some of this might apply here!

you want more from your TPI ? well heres what works.
(1) get a low restriction air filter and install it
(2)pull that plenum off, remove the throttle body then inlarge and smooth the area behind the throttle body,siamese the openings in the plenums ports too the start of the torque tubes and open up and smooth the intake ports on the entrance of the torque tubes to match.
(3)remove the screens from the M.A.F sensor(be very careful not to hurt the filament)
(4)cut the heat sink fins inside the M.A.F. sensor too a much shorter length and smooth the ends to lessen the airflow restriction
(5)bypass the heater hose to the throttle body with a single piece of heater hose or splice the two ends with a short piece of pipe, that heat only hurts your performance once the engine is up to temp.(yes this part is a bad idea if its really cold were your at!)
(6)swap to 10/30 weight synthetic oil (if you have not allready because it has more heat protection and less friction than conventional oils)
(7)while you have the throttle body off carefully clean it with carb cleaner (don,t get any on the IAC sensor or the TPS sensor)
(8)make sure your valves are adjusted correctly
(replace your spark plugs if they are over 20.000 miles old
the above changes will cost you (1) afternoons time , a set of intake gaskets and the use of a dremel/die grinder for a few hours, you will gain about 20-30 hp and cost very little!(yes it may idle a little ruffer untill it warms up because of the increased cold air entering the engine but but the difference will be something you can measure at the track. not some advertiseing jerks claim to sell something that is only marginally efffective at high rpm only!!

pablocruise

CAUTION!!!! #7 above should read "Throttle body" cleaner. Not carb :nono: Do the rest of 'em though!!


[Modified by 89 Paul in cal, 4:10 PM 10/25/2001]

TomH91

Thanks for the suggestions so far. The car already has a K&N air filter. I wouldn't be allowed to do the intake porting, but I can do the exhaust as long as it uses the cats and same hardware. There are plenty of options there. I will replace the stock ignition coil and post the results after the weekend. I think the compression issue is unlikely (but not impossible) because there is zero oil consumption. I agree that a new timing chain and gears are in order soon, and I am thinking of replacing the stock valve springs and seals at the same time. Based on the vacuum checks, the cats and precats seem okay for now. Will post again in a few days.

Beach Bum

You know things like this are one of two things... either not enough fuel or not enough spark.... with that in mind, simplfy things and start working backwards.... I noticed you said you have fuel pressure... How do you know ? Did you check at wot ? Fairly common for the pump to be fine for driving around, but when under acceleration to dump out on you. Tape a fp gauge to the windshield and go out and make some hard accelerations and watch the fuel pressure closely.

good luck
Beach

h rocks

Along those same lines, if you haven't changed your fuel filter for a while (~20,000 miles) could very well contibute to your woes. Clogged cats will wreak havoc too.

91Vette

I'm with you in your search for more power out of an 91 L98. My car seemed down on power and felt sluggish. I took it to the Chevy dealer I trust and had them do a bunch of tests. Fuel pressure was good, exhaust backpressure was fine, injectors were fine, etc.

They proclaimed my car to be in a fine state of health. What to do now ... I guess I could modify it, but I am a fan of keeping things stock. Maybe it's time for a more recent Vette. I just don't think I could part with my baby, though. That would mean I'd own two Vettes :)

vettenuts

This is interesting. Do you have a scanner? I would be curious if when it feels strong it is also in open loop and running off of the tables. Have you checked the spark knock counts and BLM values? TPS, MAT and MAP voltage?




[Modified by vettenuts, 6:04 PM 10/27/2001]

markshop

Have your mechanic check the EGR system. I just got done spending $1100 and four shops to get mine to pass emissions. The EGR selonoid was opening the EGR valve too much and leaning out the combustion gas and causing a minor miss.

85vet

When I first purchased my 85 last October, I could hardly spin the wheels, with a standard to boot. Replaced EGR, IAC, New Injectors and a good tune-up(plugs, wires, Coil, cap). I can now say I have to take it easy or I will burn the tires.

The EGR and IAC were replaced prior to the injectors and had some improvement. But the most significant increase/return of power and torque came from replacing the injectors.

Good luck,

Jim C

importeater

few things have been forgotten here & as i've spent the better part of a week under the hood of a 1990 i've done a fair job of figuring out what causes all this crap. here's kind of how my list of problems came out to finally fix my sputters spits & chings. first off the coil was nearly shot, i had no idea but did install a msd box & blaster 2 one night & all of a sudden the car ran 10x better, this whole time we were suspecting injectors when it was in spark..also advance..there's more here so lets explain. the coil did help & when the car is cold it runs like a champ, over 120deg or so things kind of come and go, what;s wrong? well chris at TPIS & i had a chat one day & i basically was saying i have the odd feeling my knock sensor is on the fritz..it's come and go and in different stages every time. he agreed with me & gave a pretty good explination of how the knock sensor worked and when they fail basically every problem i had they create. timing just goes haywire but under no load the computer sees a different map & will run fine, heavy load and race conditions however in my case were the problem and due to incorrect ac voltage thanks to the knock sensor. it sounds like a lot of people are having similar problems on similar years. it vary well could be the shelf life of the knock sensor or who knows but most issues i've read sound very much like timing events. if you have access to diacom it would do wonders as tomorrow a friend is bringing a copy over so we can drive and see exactly what's going on in the engine. also..guys..if you spin the car at all..i'm telling ya, as cheap as a msd box & outboard coil are..it's a mod worth doing. car fires in an instant and i swear it picked up hp. anyway i hope this helps as i was about pulling out my hair trying to figure this darn thing out for awhile. :chevy

TomH91

ImportEater's post was very timely - I tried to autocross the car this weekend and ended up having to retire it and complete the event in a borrowed car, an '89 6sp that pulls like a bull and it's bone stock. I replaced the coil but there was no improvement. The car has developed additonal symptoms. It pulls pretty well up to 3500 rpms, at which point it falls off a cliff: it misses and blows black unburned fuel smoke at the tailpipe. All power goes away. This is only under load, though, when you are driving the car and get into the throttle. Put the car in park or neutral at a stop and it'll pull to redline without any problems. Several of us spent some time with a shop manual and the diagnostic tree leads us to: 1) ground wire problems with ECM; 2) bad EGR; 3) bad ECM; or 4) bad knock sensor. Or, this may turn out to be a gasket problem as suggested above. There is a pronounced whistle on acceleration from somewhere under the hood. I have not been able to locate it and I have tried to check all the vacuum lines I could identify, but could not find any which were cracked, split underside, abraded, etc. There are still no codes. I do not have access to a diacom or other scan tool, so it looks like I have to resort to a dealer. There are some Corvette shops in town, but none are recommended. Thanks to one and all for your suggestions and help.

DOCTOR J

FWIW, I've had my '91 since it was new and do an occasional autocross & day at Limerock.

Earlier this year I installed a SR intake, and wound up spending quite a few hours with Diacom this summer, tuning a new chip to match the SR.

(BTW Diacom or some other scanner will save you a ton of money compared to going to a dealer for diagnostics - generally speaking. Check out the Craig Moates software on the 3rd Gen DIY PROM board - it's free, and I believe it works with the 727 computer now.)

Even after years of careful maintenance, when I took a thorough look at mine under the Diacom microscope there were a number of parts that were worn, or weak enough to affect performance:

To get good baseline runs I invested in a new knock sensor, O2 sensor (that it needed for sure), MAP sensor (OK, I broke the old one), IAT sensor, Distrib cap & rotor, plugs & wires.

For the sake of being absolutely certain everything was up to snuff I also replaced the spark module and ECM (I don't know that they were bad, but they were more than 10 years old so it's a judgement call) and cleaned all the contacts.

I did find that the EGR valve was sticking - this gives you major mixture problems. Replaced that and the solenoid. Also found that at least one injector (haven't tested them all yet) was clogged - which ran one bank lean and resulted in a bunch of knock retard. Replaced all with SVO's.

It runs well now, though I'm not finished tuning the chip yet. Now that boating season is over I'll get back to a little more tuning work before it gets too cold. (I'm building one of the DIY WB O2 sensors to see if I can get a better handle on WOT spark and fuel with these wierd injectors.)

HTH

drj

Aggravated4life

It almost sounds like the cats are clogged.
When mine were clogged up,after 3,000 rpms or so it was a dog.Replaced them and it ran harder then ever.Black smoke is also a sign of excess fuel.Either too much gas or the computer turning up the gas because of a vacuum leak somewhere.

Good Luck and keep us posted.