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L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

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Old 02-16-2009, 01:39 AM   #1
janarvae
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Default '91 L98 Hard Start, Stalling, and Running Rich

Hi all,

I've posted about my issues as I've brought them up in different threads. I wanted to bring everything together, into one, now that I've done some more datalogging and went through some FSM trouble-shooting charts. This is for a 1991 L98, with about 86,000 miles.

I have had an intermittent problem with a Code 42, hard start, and rich-running.

Code 42:
I recently reset the base timing to 6*BTDC. The EST connector is well seated and the trouble codes have been reset several times. The Code 42 keeps coming back though. New coil and cap were recently installed.

Hard Start:
It will crank for about 4 seconds and not start. I'll try again, and get the same result. Then, usually, on the 3rd try, I'll get a start. It will start the first time with my foot on the accelerator though. If I don't keep my foot on the accelerator, it will stall. The idle shakes the car now!!! New Bosch Design III injectors were installed recently.

Running Rich:
The fumes are unbearable at start-up and idle. You can definitely smell the unburnt gasoline. The tubing just behind the passenger stock pre-cat is turning red after running for 10-15 minutes. The excess heat from this recent occurrence has burnt a hole through the carpet padding and smoked-up the cabin before I discovered the problem. I recently removed the front y-pipe, back to the mufflers. I did not hear any rattling when I shook the precats or main cat. I am also getting very poor gas mileage (possibly exacerbated by a failure for the TC to lock-up).

I am thinking the rich condition is causing the precat to overheat on the passenger side? Or could the precat be failing, causing a rich situation?

Observations/Testing:

One of the first things I tested was fuel pressure. With the ignition to 'On', the fuel pump ran for 2 seconds. This indicates a good fuel pump relay?

I hooked up a fuel pressure gauge and observed 42 PSI at 'On' position of ignition. I left it on there for 2 hours and came back. The pressure had fallen to 10 PSI. However, I had initially waited 10 or so minutes and there was no loss of pressure from 42 PSI. I have new Bosch Design III injectors on.

While idling, the pressure is at 35 PSI, within normal range.

I added a bottle of Iso-HEET to remove any water that may have collected in the tank while it sat for 6 months.

I also checked the MAP sensor. With ignition 'On', MAP completed connected, read 4.90V. With engine started and MAP vacuum plugged and MityVac connected (with no vacuum applied yet), it was at 4.86V. With 10Hg vacuum applied, while idling, it was at 3.04V. This is a negative difference of 1.82V. The FSM was not clear if the result between 10Hg vacuum and no vacuum applied should be positive, not negative.

I reset the new IAC again via the FSM procedure.

At idle, in Park (auto) here is some data:
BLM - 138 (150 is considered lean, 108 is rich)
Block learn - 4
IAC - 50 steps at 800 RPM. 36 steps at 725 RPM (81*C temp, running for 391 seconds, 2 knock count)
0 Degrees knock retard.
O2 Sensor - Varied from 800-150mV
Fuel Pump - 14.20V
Battery - 13.9V
Injector Pulse Width - Varied from 2.27-2.33mS
Target AFR - 14.73

So, what could be causing the rich condition and POOR gas mileage? What could be causing the slow loss of pressure in the fuel rail over 2 hrs? Is the MAP sensor still functional?


Thank you,

Jonathan
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Old 02-16-2009, 10:05 AM   #2
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ttt
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Old 02-16-2009, 02:29 PM   #3
RandyJ75
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Check out your Coolant Temp Sensor. If you can scan the car, check and see what it is reading. Make sure it does not have any teflon tape around it, and that it is grounding to the manifold.

Read this: http://forums.corvetteforum.com/c4-t...me-help-2.html


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Last edited by RandyJ75; 02-16-2009 at 02:42 PM.
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Old 02-16-2009, 03:19 PM   #4
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Jonathan,

I'll take a stab at it:
Clearly there is something going on the right bank of the engine/exaust. Your o2 finding shows that the car is seeing lean yet the car smells like it is running rich. You stated that you removed the Y pipe and shook it but nothing was loose inside.....so did you just bolt it back on the car after that?

Code 42 indicates that the ECM saw voltage on the EST line (white wire between ecm and ignition module) when it should have been grounded by the ignition module. Could be poor connection, a faulty ignition module (in the distributor), or a defective ECM.

The hard starting could be caused by a combination of problems. Here are a few things I would do:
#1 I'd get that old "y" pipe out of there before the car burst into flames and install one of these "STAT" (medical lingo):http://www.mamotorworks.com/corvette?frame=3.971 Your car will still be enviro friendly because the main cat still in place.
#2 Pull the vacuum hose of the side of the FPR and check to see if fuel is blowing out the fitting while the fuel pump is running.
#3 Check your distributor wires for secureness correct placement on the cap and that the distributor cap is fully seated against the bottom half of the base.
#4 Get your timming light out an strap the pickup clamp around the plug wires on the passenger side to make sure all the plugs wires are getting voltage from the distributor/coil.
#5 Pull the spark plugs on the passenger side to make sure they are all in good shape and look like they are getting spark across the tips and are burning cleanly.
#6 If you still have problems remove the ICM have it tested or replace it with a known good one. Don't forget to pick up some "heat sink" compound to install the ICM. The dielectric grease will/would work for awhile until it gets hot then it will dry up/run out and not transfer the heat properly to the distributor body.

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Old 02-17-2009, 05:52 AM   #5
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My 1990 did this a new CTS and magnetic pickup coil cured it. BTW my car never threw a code even though the new cts made the car start cold without pumping the gas pedal.
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Old 02-17-2009, 10:26 AM   #6
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Thank you all for your replies thus far. Yesterday, I decided my cat was plugged and drove it over to a local exhaust shop. On the way there, I almost overheated for the 1st time- I pulled over and waited for about 20 minutes after the temperature rose to ~250 and kept increasing. When I finally arrived at the exhaust shop, they used infrared thermometers to check the temperatures of all 3 catalytic converters. They said all of them were bad. How true this is, I'll never know, but I do know both footwells are considerably cooler, the car is getting better MPG, and the coolant temperature gets nowhere near the danger zone with a stock temperature thermostat, new water pump, and recently flushed and filled proper green coolant.

What they did was removed the three cats and welded in test pipe in their place. They cut the flange off of the main cat and welded it to the test-pipe so my Corsa cat would still bolt on. I said, "NO," to welding the Corsas! The whole job set me back $250. Exhaust work is new to me, so tell me if I got ripped or not, because I plan on letting them attach my LT headers (when I buy them) to some new bullet cats.

The reason I allowed them to remove the cats was the overheating issue needed to be resolved immediately, and I could not wait for the MidAmerica y-pipe or for them to order a main cat for the car. However, I will be purchasing some LT headers soon and will install bullet cats or a single, high-flow cat at that time.

The volume of the exhaust is increased DRAMATICALLY! Remember, this has stock manifolds but then is pipe all the way to the Corsas. It is almost too loud at WOT! Seriously. At idle, up to about 2,000 RPMs, it is quiet. However, when I open it up to 3,000 or so, it is SCARY! It actually hurts my ears and my friend, who drove behind me, said it scared her and was VERY loud and audible even after she turned down another road and I continued in a different direction. I will post a video comparison of the stock exhaust, stock exhaust with Corsa cat-back, and stock manifolds with pipe back to the Corsas.

Quote:
Originally Posted by RandyJ75 View Post
Check out your Coolant Temp Sensor. If you can scan the car, check and see what it is reading. Make sure it does not have any teflon tape around it, and that it is grounding to the manifold.

Read this: http://forums.corvetteforum.com/c4-t...me-help-2.html

Randy
I installed a new AutoZone CTS when I changed my intake gaskets about 2 months ago. Problem is, I believe I applied some teflon paste to the sensor. I'll go and double check. The gauge in the cluster and my laptop are reading the same.

Quote:
Originally Posted by engle1147 View Post
Jonathan,

I'll take a stab at it:
Clearly there is something going on the right bank of the engine/exaust. Your o2 finding shows that the car is seeing lean yet the car smells like it is running rich. You stated that you removed the Y pipe and shook it but nothing was loose inside.....so did you just bolt it back on the car after that?

Code 42 indicates that the ECM saw voltage on the EST line (white wire between ecm and ignition module) when it should have been grounded by the ignition module. Could be poor connection, a faulty ignition module (in the distributor), or a defective ECM.

The hard starting could be caused by a combination of problems. Here are a few things I would do:
#1 I'd get that old "y" pipe out of there before the car burst into flames and install one of these "STAT" (medical lingo):http://www.mamotorworks.com/corvette?frame=3.971 Your car will still be enviro friendly because the main cat still in place.
#2 Pull the vacuum hose of the side of the FPR and check to see if fuel is blowing out the fitting while the fuel pump is running.
#3 Check your distributor wires for secureness correct placement on the cap and that the distributor cap is fully seated against the bottom half of the base.
#4 Get your timming light out an strap the pickup clamp around the plug wires on the passenger side to make sure all the plugs wires are getting voltage from the distributor/coil.
#5 Pull the spark plugs on the passenger side to make sure they are all in good shape and look like they are getting spark across the tips and are burning cleanly.
#6 If you still have problems remove the ICM have it tested or replace it with a known good one. Don't forget to pick up some "heat sink" compound to install the ICM. The dielectric grease will/would work for awhile until it gets hot then it will dry up/run out and not transfer the heat properly to the distributor body.

I'll check the spark plugs and wires, ignition, grounds, CTS, and timing this weekend. As of now, my overheating and rich condition are no longer present. The exhaust definitely smells different with no cats, but it doesn't smell rich. I also cannot see gas coming out of the exhaust now. However, the idle is still pretty shaky, so I'm not out of the woods yet, so to speak. Thanks!

Quote:
Originally Posted by jeffp1167 View Post
My 1990 did this a new CTS and magnetic pickup coil cured it. BTW my car never threw a code even though the new cts made the car start cold without pumping the gas pedal.
Seems like the consensus is a bad or improperly installed CTS. I'll check in a couple of minutes and check the pick-up coil later. I transferred the stock coil to a new Delco distributor cap during my intake gasket swap, along with a new distributor rotor.

Thanks for all the suggestions,

Jonathan

Last edited by janarvae; 02-17-2009 at 10:29 AM.
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Old 02-17-2009, 02:08 PM   #7
SunCr
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Rescan in Closed Loop to obtain Block Learn and Integrator. If it's still lean, air of some other substance is getting in and tricking the O2. That causes the ECM to add more fuel than it needs so it runs rich. In light of the shaky idle (and the fact that it's an L98), I'd want to rule out a head gasket leak at #7. Even small amounts of coolant or moisture is going to lean it out. Otherwise, vacuum, exhaust, manifold leaks or any air after the throttle body is the usual culprit though you generally don't get severe symptoms on a Speed Density system (as opposed to a MAF).

You check the CTS by checking it cold; ie, after it has sat overnight. It should read ambient (or close to it) as should the Air Temp Sensor.
From what you've posted, I don't see a problem and teflon tape has nothing to with anything. The Sensor is grounded by the Black wire which is spliced into the TPS and Air Temp Sensor ground and then back to the ECM. The ECM grounds are at the Bellhousing. In any event, if there was high resistance in this circuit, it would show up in the other sensors.
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Old 02-17-2009, 02:24 PM   #8
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PS - recheck your MAP voltage - it should be about 1.5 volts at idle; 4.5 volts or more, key on engine off. From your data, you probably need to troubleshoot this first looking for a vacuum leak in the supply line or simply replacing it.
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Old 02-17-2009, 02:36 PM   #9
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Quote:
Originally Posted by SunCr View Post
Rescan in Closed Loop to obtain Block Learn and Integrator. If it's still lean, air of some other substance is getting in and tricking the O2. That causes the ECM to add more fuel than it needs so it runs rich. In light of the shaky idle (and the fact that it's an L98), I'd want to rule out a head gasket leak at #7. Even small amounts of coolant or moisture is going to lean it out. Otherwise, vacuum, exhaust, manifold leaks or any air after the throttle body is the usual culprit though you generally don't get severe symptoms on a Speed Density system (as opposed to a MAF).

You check the CTS by checking it cold; ie, after it has sat overnight. It should read ambient (or close to it) as should the Air Temp Sensor.
From what you've posted, I don't see a problem and teflon tape has nothing to with anything. The Sensor is grounded by the Black wire which is spliced into the TPS and Air Temp Sensor ground and then back to the ECM. The ECM grounds are at the Bellhousing. In any event, if there was high resistance in this circuit, it would show up in the other sensors.
I'll check the temperatures tomorrow morning after it has been sitting for awhile.

Speaking of the ECM grounds, I broke the star washers on the passenger side of the bellhousing and cut the wires and spliced new ones in with ring terminals. Then I placed shrink-wrap over them.

Old Ground:

Click the image to open in full size.

New Ground: (I later put heat shrink over each crimp connection and a large piece over the entire length).
Click the image to open in full size.
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Old 02-18-2009, 10:28 AM   #10
janarvae
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Default As promised....

Stock:



Corsa:



Corsa w/out cats: (intentionally blurred and darkened)



Tell me what you think....
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Old 02-18-2009, 03:14 PM   #11
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I think it sounds good. It is much louder then before, but so what? How does it sound going down the road? When I put dual exhaust on my car, it did not have cats, and sounded just like yours. I had to install cats so it would pass emissions, and they made it much quieter.

Randy
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Old 02-18-2009, 03:29 PM   #12
janarvae
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Quote:
Originally Posted by RandyJ75 View Post
I think it sounds good. It is much louder then before, but so what? How does it sound going down the road? When I put dual exhaust on my car, it did not have cats, and sounded just like yours. I had to install cats so it would pass emissions, and they made it much quieter.

Randy
Thanks Randy,

It is LOUD when accelerating at 2,500 RPM and above, but pretty quiet at idle and light throttle/cruise. I'm going to put on some cats again soon when I buy some LT headers. Hopefully the headers will counter-act any muffling the cats create. And it sounds like the end of the earth going up the ramp in a parking garage.

-Jonathan
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Old 02-18-2009, 03:29 PM
 
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