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!

 

I'm sure you've tried but that sounds ALOT like my 93 when the Idle Air Control module/sensor went out...

have you tried to spray it off with some carb/intake cleaner?

 

Yup, the IAC is brand spanki'n new!

And yeah I've got 4 pages of things I've checked/tried/replaced....

Unfortunately, I don't hink this is gonna be an easy one...

 

Email me your Datamaster files. I don't have the 1993 version, but might be able to see something. I could always look at them as csv files, but prefer to view them with Datamaster.
No guarantees .

 

How about a bad vacuum leak somewhere, wouldnt that cause something like this, I of course mean a BAD leak.

 

I say maybe the coil,or coil module.(Check harness).Please post when fixed.Post pics of the Goat.Sounds like a sweet ride.My02

 

I'll try and post the [4 -page] list of things I've tried......

 

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)

 

Do you hear any wierd noises? Could have a hole in one of the pistons check the oil for strong smell of gas, while engine is off unplug a spark plug wire and then start the engine, see how it runs, if same move to next wire, repeat the steps till the engine sounds different and that may be your bad cylinder/piston.
You have done everything except blow it up and I am sure you already checked all leads and found them solid and checked all fuses\replaced bad ones Also could be a bad camshaft/wiped lobe
take the oil filter off and cut it in half-check for debris or shavings in it.

 

This may sound a little weird, but do you have a coolant temperature sensor screwed into the front of the waterpump housing? Your initial symptoms sound similar to the problem i was having before. If you do have one, try and change that. It's only like an $11 part. I fought my problem for about 2 weeks and a friend suggested i change that, after i did, it fired up and hasn't had a problem since. Good luck, and let us know how it turns out.

 

Do you have a cat on the vehicle?

 

Possibly a bad fuel pressure regulator - Have you checked the pressure on the rail? This could cause your rich condition

Possible bad MAP sensor - if it's at all faulty, the computer will not calculate fuel correctly. The 93's are more sensitive to bad MAPs since we don't use a MAF to measure how much air is comming in.

Send me your DM files and I'll have a quick look for you.

 

I can only look at the DataMaster files if you have them in .csv format. I can give you an e-mail address to send them, if they are in that format.

The things that stand out are the irrational values you get with the timing light (you should only see 18-20deg at idle , and yes, there is no reason you should see 60-deg), the 10"Hg vacuum, and the 160 IAC count. Have you checked the valve adjustment? Have you verified lift on the cam (badly worn lobes)?

Maybe I missed it in your list, but have you ever done a compression and/or leakdown test?

 

I was also going to suggest checking the valves.

Perhaps the rockers are set too tightly. You could at least check them with the engine running.

I believe the quick/easy method is to tighten until they stop making noise then back off about 1/4?

 

Initial thoughts:

There is a large amount of fuel being called for at idle. This is likely driving the IAC counts up. The source of those things is probably because the MAP reading quite high. The sensor could be causing this or the reading could be high because of a physical condition that is causing low vacuum.

You say the EGR is not leaking. What about the possibility of an intake gasket leak? Some people spray various "potions" around the intake to try to test for leaks (plus the valvetrain ideas that Injuneer mentioned).