goatcrazy

I've been working on this problem for 1.5 MONTHS now, & I'm getting desperate! The car is a '93 LT-1 corvette that barely idles and dies when you give it gas. I bought the factory service manual, and the Datamaster software, and have checked every pertinent sensor, and have follwed all the GM troubleshooting guides and CANNOT solve the problem! I've got a LONG list with the problem history and the things I've tried, along with 3 datamaster scan files that I can e-mail to anyone that can help.



This has gone on WAY too long now, and I really need some expert help on this matter, I can't think of anything else to try!

I don't want to take [tow] it to the dealer and have them run up a tab learning how to work on the car! And I don't really know any LT-1 experts in the area.

PLEEEEEEESE Help!

Thanx again!
:banghead: :banghead: :banghead: :banghead: :banghead:

goatcrazy

O.K. Here's the complete list. It's pretty long..

Test log & results datasheet

Vehicle: ’93 Chevrolet Corvette
Engine: LT-1, w/Speed density, 1st gen opti
Trans: Automatic w/ASR {automatic slip reduction}
Mileage: 178,000 – All original powertrain

Background

I just recently purchased this car 1 ½ months ago. It’s been in and out of the shop for a number of months. Here’s the chronology:
10/8/02: Complaint: Engine runs rough, drop in MPG. Brought to dealer, dealer troubleshot, replaced spark plug wires.
2/17/03: Woke up one morning car wouldn’t start. Brought to dealer, replaced optispark, and water pump. Also replaced shifter cable {binding}.
5/22/03: Woke up one morning car wouldn’t start. Brought to dealer, replaced coil. Also replaced fuel filter.
6/22/03: Woke up one morning car wouldn’t start. Brought to dealer, dealer completed electrical check, found nothing wrong. Suggested replacing timing chain. Vehicle pulled from dealer, sat for two months, then sold to me on 9/10/03.

Current condition:

Vehicle will start, idles for a short while then dies. Idle is incredibly rough. After a re-start with gas, the car will idle {rough}. If accelerator pedal is touched it dies. There is no black smoke coming from the exhaust, idles around 500 rpm. There are no check engine codes being set. The computer will go into diagnostic mode. I have a factory service manual for the car, and have been following the factory troubleshooting procedure for “Rough, unstable idle, stalling” AND “Hard start”. Also have purchased TTS DATAMASTER, and have several scan files.

CHECKS:

Electrical

1) TPS SENSOR: Voltage output at sensor: 0% throttle - .62V, 100% throttle: 4.2V, linear transition. DATAMASTER: 0% at 0% and 100% at 100%, linear in between.
2) MAP sensor: Voltage output at atmosphere at sensor: ????????, with 29” vacuum applied to sensor: ????????, on DATAMASTER: atmosphere: ???????, 29” vacuum applied to sensor: ???????. Map sensor gasket on & installed properly. [Sensor readings were in-line with service manual, I didn’t write them down]
3) ECT sensor: On DATAMASTER: 75deg at ambient, increases linearly with engine runtime.
4) IAT sensor: On DATAMASTER: ~75 deg w/ambient at ~70 degrees.
5) IAC valve: Replaced IAC valve. On DATAMASTER: IAC usually STAYS at 160 counts. Performed IAC reset procedure, IAC went to 12 and came back quickly to 160????
6) Checked injector resistance: all injectors 12.6 +/- 0.1
7) Checked PNP switch circuit. Circuit input to ECM electrically ok, DATAMASTER shows input operating correctly.
8) Checked EGR solenoid, solenoid holds vacuum, is NOT switched on at idle. Output OFF in DATAMASTER.
9) Checked AIR system: Fuse was blown for AIR pump. Replaced fuse, AIR pump started working – no change in condition. Checked operation of check valves – Worked properly.
10) Injector fuses are ok
11) Cleaned all engine compartment grounds.
12) Checked w.ohmmeter all ecm grounds. All were < 1 OHM.
13) Pulled all spark plugs. All plugs were black w/heavy gas smell. Replaced all plugs with ones that were known good.
14) Replaced ICM module.
15) Replaced IGN coil (again).
16) Checked ECM output to ICM – Varied between 1-4 volts.
17) Generator output ~ 13.4 volts.
18) Checked #1 spark plug output : snappy blue spark.
19) Ohmed out all spark plug wires, al wires going to correct cylinders, and was of an appropriate ohm reading {1.5K – 5K based on length}
20) When I replaced the timing chain, I made marks on the balancer & OPTI for #1 TDC. Put a timing light on, and noticed timing was at 80 deg advanced, computer was calling for 32 deg. Pulled opti, disassembled, rotor & points were immaculate, optical section immaculate as well. Placed opti rotor at #1 TDC. Made sure key mated properly with cam gear, re-checked timing, was at ~30 deg, when engine is revved, jumps to 60 degrees, then back to 30, computer does not show a call for 60 degrees, but maybe sample rate is too slow? [No net change in condition]
21) Today, I got desperate and bought a new PCM. This, of course, had no effect.

Mechanical

1) EGR valve: Valve was not mechanically open at idle. Did NOT hold vacuum on air diaphragm. Replaced EGR valve.
2) Replaced PCV valve and hose. Found crack in PCV hose.
3) Replaced all vacuum hoses.
4) Pinched off all vacuum hoses one by one to check for vacuum leaks-no change.
5) Checked operation of EVAP canister purge solenoid. Solenoid valve holds vacuum, operates correctly, is not on at idle. Replaced hoses.
6) Checked fuel pressure: 43 PSI key on, 38 PSI engine running. NO drop at all after 10 minutes ignition off.
7) All injectors are “clicking”. Unplugged injectors one by one, Idle got rougher each time one was unplugged. {as if a cylinder was dead}
8) Replaced timing chain. There was timing chain slop, but the timing had not “jumped time” sprockets lined up “dot to dot”.
9) Drained all the fuel from the gas tank. Applied 20 PSI compressed air to fuel press line to fuel rail, bled fuel out at fuel rail schrader valve. Wiped gas tank down by hand, replaced with fresh gas. No difference.
10) Checked for vacuum leaks: Sprayed carb cleaner around intake manifold, listened with tubing, etc. Found nothing.
11) Removed throttle body. Cleaned all passsages with brush. Cleaned throttle butterflies with soft cloth ONLY. Cleaned IAC passages. Replaced throttle body-Intake manifold gasket.
12) Pulled both valve covers: All rocker arms moving full travel, no tapping, etc.
13) Measured engine vacuum: 8-11” Hg. {Because of bad idle?}
14)

Other tests/observations

1) IAC valve is always at 160 counts.
2) Fuel trim values long term is almost always at 120, short term in closed loop varies up and down but seems to be in control.
3) “Service ASR” light comes on occasionally. Also passive keyless entry light is on all the time. [I have no idea how to retrieve those codes w/o the “tech 1” POS.
4) I varied the output of the map sensor to the computer by applying vacuum, and releasing to go from atmosphere to full vacuum to see what effect it would have. The engine was only happy at actual vacuum condition 10” Hg. Stalled otherwise.
5) Disconnected TPS from butterflies. Operated TPS independent of butterflies, and vice-versa (in an attempt to “fool the computer” into changing the air-fuel ratio). This really had no effect, I’m not really sure it did anything because of the map sensor’s input.
6) Manually operated EGR valve. Engine stalled {as it should}.
7) Interesting test: [ See datamaster log #3] If I oscillate the throttle quickly, I can get the engine to rev up to 2500 rpm with no backfiring, stalling, or anything. WHY?
8) The computer does not turn on the primary cooling fan on at 218 degrees like the manual says. It turns on at 238 when the SECONDARY should turn on. Both fans are working because when I put the computer into diagnostic mode, both fans turn on. Also the fans come on when the A/C is on.
9) The exhaust doesn’t smell unusual.
10) The datamaster shows “injector faults” on both bank1 & bank 2 injectors [in the bit status display]. Don’t know if that’s real or a software bug. No codes are being set.
11) Removed oil filler cap. Idle did not change. {High crankcase pressure?}
12)

Nathan Plemons

I haven't read everything yet, but send me datamaster files and I'll take a look. Can't make any promises, but sometimes just another set of eyes can see something different.

Nathan.Plemons@wku.edu

old salt

the iac counts sound very high. 160. i usually dont see over 60. try using carb cleaner and spraying into tb while idling. if idle comes up and improveds you are looking at a fuel problem. i really dont understand your timing explanation. i am not aware of using a timing light on an opti lt1. the crank pulley isnt really set upp for that. i have a 94 and i have never used a timing light on it.



[Modified by old salt, 2:15 PM 10/8/2003]

bogus

there is a ton to read... whew...

I would have to make sure the IAC circuit is not somehow compromised. A short, bad connector, something silly like that.

these problems sound like a problem with the opti data. The opti has a diagnostic port on the right side of the engine, make sure that the connections there are clean and free of corrosion.

have you replaced the knock sensors? if not, I would try it... that could explain the wacked timing readings.

what about the intake gaskets themselves? I doubt they would cause this, but who knows....

Stupid thought - are all the plug wires tight????


[Modified by bogus, 2:20 PM 10/8/2003]

ittlfly

Use a DVM on the injectors to check the solenoids. They should all be within 1 ohm of each other.

Goody

Have you done a leak down compression check? With 178K miles the rings might be shot.

I replaced an ignition control module with one from autozone. Turns out it was bad. Try it on another car and see if the condition moves with the part. I was forced to buy one from the dealer.

As bogus said, check the opti plug on the right side of the intake under the fuel rail cover.

Don't try timing light stuff on LT1 cause it doesn't work. There isn't really a reference point to go by.

What data is the O2 sensors putting out?

One more thing to check. Gently tug on the IAC senor plug wires. I had a loose wire inside mine once. Make sure the wires are making good contact.
I have to think it is in the idle circuit somehow because of those IAC counts being so high......Are you sure there isn't an intake vacuum leak somewhere?

Have you checked the EPROM? I know you replaced the ECM, but you are using the same EPROM chip. Any chance you would have access to another one just to check that?


[Modified by Goody, 3:37 PM 10/8/2003]

1_piece_at_a_time

Send me the data files you have and I'll take a look, too. If it's barely idling, that would explain the IAC count being at 160. The ECM is trying to idle the engine up by backing the IAC all the way out. Once you get a little past idle the IAC isn't really a factor so I think the problem is elsewhere. Between all of us, I'm sure we can figure this one out.

:grouphug:

Del96lt1

THis may sound really crazy, but.... Did the guy you bought it from have any enemies ???? Remember the old potato in the tailpipe trick??? You may want to try disconnecting the exhaust. Maybe plugged cat ????

grapeknutz

A shot in the dark, it sounds like a timing chain gone bad.

bogus

new chain... perhaps it's off a link?

Mr Mojo

Of everything you checked or replaced, I didn't see Optispark mentioned.

How long has the current one been in there? With 178,000 miles I hope it isn't the original.

I only say this since I had similar symptoms in my truck with 189,000 miles. I had to rebuild the distributor.

Opti spark is a greta piece, but it won't last forever just like a cap and rotor on a standard ignition won't last forever.

If I missed the part where you mentioned optispark, I apologize, but I would definitely check that.

Goody

Mojo, he has it as being replaced on the list by the dealership at the same time as the fuel filter was done.

My question is: Was it the whole Opti unit, or just a cap and rotor job??

bogus

good point... but then he would get a code 16 from the bad opti side...

I have heard of some nasty opti's that will at least run...

I do remember a thread from a LONG time ago... a guy had an Fbody that would not run... the final cause? an oil leak flooded the opti... just an idea.

however, I think the focus should be spent on the IAC... that circuit, considering age and mileage, is a good place to start!

Goody

Okay, but he says that as soon as you touch the throttle the car dies. That isn't the IAC circuit is it?? The TPS is putting out the correct info from what he describes.....What about the power enrichment or MAP not working correctly? I'm throwing ideas out to see if it jogs memories.

My car was idling like crap and surging a day ago from my LTCC experience. I took off my new MSD 8.5 wires and put the factory wires back on and it runs perfect now.....go figure.

goatcrazy

Timing light question "Don't try timing light stuff on LT1 cause it doesn't work. There isn't really a reference point to go by."

Yes, you're right normally, but when I replaced the timing chain, I made match marks on the balancer and the opti face for TDC, so I could check timing in the future (THANK GOD!).

"What data is the O2 sensors putting out?"

They are varying from .1 -> .9V (rapidly) and from the short term and long term fuel counts the system appears to be responding, althought the long-term is always rich.....

"Have you checked the EPROM? I know you replaced the ECM, but you are using the same EPROM chip. Any chance you would have access to another one just to check that?"

Yes, I replaced both at the same time...

"Have you done a leak down compression check? With 178K miles the rings might be shot."

No, I havent, I've honestly stayed away from the mechanical things based on the pattern of no-starts and electrical fixes...

But at this point, I plan on doing one...

"One more thing to check. Gently tug on the IAC senor plug wires. I had a loose wire inside mine once. Make sure the wires are making good contact.
I have to think it is in the idle circuit somehow because of those IAC counts being so high......Are you sure there isn't an intake vacuum leak somewhere?"

That's a good point about the IAC wiring, I should ring out those wires from the ECM to the plug...I'll try that 2-nite! I tried spraying carb cleaner around the intake and nothing happened..Also listened with a hose....Found nothing..

goatcrazy

Of everything you checked or replaced, I didn't see Optispark mentioned.

Yes, the opti-crap was replaced 6 months ago...

goatcrazy

"Use a DVM on the injectors to check the solenoids. They should all be within 1 ohm of each other."

Yes, I did that check and they were all 12.6 +/- 0.1 ohm

goatcrazy

"My question is: Was it the whole Opti unit, or just a cap and rotor job??"

Last weekend I took the opti off and completely dis-assembled it and it looked like it was spankin' new....

1_piece_at_a_time

Does the RPM you read on the scan tool seem to be right? I've had a few vehicles with distributors that send faulty signals that the ECM interprets as more RPM than what the engine is acutally turning. It's a bit of a longshot, given that the optispark is new. But it's a start.

I couldn't get the data files to open. So I'm working without the benefit of your scan tool data. I'm good with cars, but not too good with PCs.

I'm also looking at wiring diagrams to see if the ASR, PKE, and the engine have anything in common. The PKE and Theft deterrent use the CCM. I'm wondering if the CCM's fuel enable signal could be causing this. I'm just throwing out ideas. You can get codes out of it manually according to my service information. I'm trying to figure out how to get this stuff copied and sent to you so you can see if there's any theft deterrent problems.


[Modified by 1_piece_at_a_time, 1:51 PM 10/9/2003]