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New to the forum here and seeking help from the LT1 experts.
I bought a 94 Z28 with a T-56 recently from a teenage guy. The odometer reads about 210k miles, but he said the PO rebuilt the engine and transmission about 80K ago. The engine is stock and has a K&N CAI and straight pipe exhaust. The car was not running when I bought it. It would only crank, but would not run.
The guy advertised it as having a "blown valve", but said he had replaced the water pump; in the process he got the Opti wet and ended up replacing the Opti with one he bought from Ebay. It started once and never started again.
Once I got it home I replaced the ICM and Coil because there was no spark. It fired up and I let it ran for 10 seconds. Then I turned it off and went to finish reassembling the air intake. Once done I tried to start it but it would only crank and no start. I bought and oscilloscope and found the Opti was not sending a signal to the ECU. So I bought an Opti from Advance Auto Parts because decided I wanted to at least get it running to see how it ran before investing good money in it. The car started up and has started ever since.
I topped off the radiator with coolant, let it ran until it sucked some of the coolant and added more to top it off and then was done with it.
I took it for a a short test drive and when I got back it was leaking a little bit of coolant, drops were coming out of both ends of the bottom part of the radiator.
Ever since I take it for about 20 min drives and have identified the following issues:
1. Recently when I drive it under hard acceleration (WOT) there are times that after shifting at around 5500 rpms to 2nd gear, the rpms drop to around 2500 and it feels sluggish and when I get to around 3000-3500 rpms it acts like it got its power back. There are other times where there is no loss of power and it pulls hard through 1st and second gear and might exhibit the issue after shifting to 3rd gear or not at all.
2. The last two times I have taken it for a drive, I have driven it hard a couple of times during the drive to see if the issue is still there, and when I get home, or stop at a gas station, it starts leaking coolant. The puddle seems to form mostly on the driver side close to the bottom part of the radiator. I started it on my driveway today and let it run at idle for about 5 min and the temp went past halfway, so I will be adding more coolant.
3. When I shift to 5th or 6th gear anywhere between 45-60 MPHs and get on the gas, the whole car shakes while accelerating in such a way that I want to increase the speed. If I only accelerate it to maintain the cruising speed, then it does not shake. The car also feels a little shaky at idle while standing still sometimes as well.
Number 3 along with number 1 leads me to believe it might be misfiring. I tried disconnecting the injector harness one by one, but honestly could not hear much of a difference with the straight pipes unless I disconnected two at a time, but when disconnecting only one, I did not hear much of a difference in how the engine sounded at idle. I checked the harness with a Noid light and all injectors are getting a signal. The car has a "trap door" so I believe the fuel pump might have been replaced.
The SES light turns on after it runs for a few seconds (as well as the ABS INOP light). I ordered an ALDL cable from Ebay and am waiting to get it before I can pull the codes. I probably will not get it until next week.
I am going to replace the fuel filter as well since I do not know when was the last time that was replaced.
Is there anything you would suggest to check as far as the misfire is concerned before I get the ALDL cable and am able to check the codes?
I am starting to think that maybe the guy was right in saying it had a blown valve and maybe has a dead cylinder or something. Since this is my first LT1, I do not have anything to compare it to to determine if it is running at full power or not. I tried the dollar bill/piece of paper on the exhaust tip, but the tip is rectangular and if I put the paper on one end of the rectangle (because the tip is too long to cover both ends) it moves outward, but when I move it to the other end of the tip, it flaps. I used a Lowe's receipt folded in half...lol.
As for the coolant leak, could driving it hard somehow build up pressure and cause it to leak, or could it be something straight forward as a leak in the radiator or one of the hoses?
Any help is appreciated and thanks for reading through the long post. I will try to post a video of the engine running in case that helps. I believe I hear some noise coming from the driver's side valve covers (maybe lifters issue) but since I am not too familiar with the LT1 I am not sure if it is normal.
- when you filled the radiator, did you bleed the air out of the system using the two brass bleeder valves? The LT1’s “reverse flow” cooling system will overheat if it isn’t bled correctly. The radiator leak should be addressed. That's not helping the coolant system. You have to keep it full, and free of air, or you're going to blow a head gasket.
- the shift RPM drop from 1st to 2nd doesn’t compute w/ the 1994 and up T56 gear ratios. 5,500 RPM in 1st, when shifted to 2nd should only drop the RPM to about 3,600-3,700 RPM. But you indicate it drops to 2,500 RPM?
- 5th (0.74;1) and 6th (0.50:1) are steep overdrive gears. Attempting to accelerate at low RPM creates huge cylinder pressures and requires higher ignition voltage at the plugs. Any weakness in the Ignition system will show up under those conditions. Can also cause detonation (knock) causing the PCM to pull timing to eliminate the knock.
When you get the ALDL cable, what are you using to scan the PCM?
The rectangular exhaust tip is not symmetrical. The exhaust pipe enters on one side, and flows straight back. The other side of the tip angles off to one side, so that may be enough to produce differing results on the two sides of the rectangle.
Looks like I came to the right place! Thanks for the prompt reply.
- I followed a guide on shbox.com on how to refill the cooling system after reinstalling the water pump. I believe I did use those bleeder valves on the small hoses coming out of the top of the water pump, but I do not remember exactly. The weird thing about the coolant leak is that I drove it probably 5 times for short 10-12 mile 20-25 min drives in the area where I live and there were not leaks when I got back and shut the engine off; and it did not overheat either. I only noticed it leaking when I first filled it after changing the Opti and in these last 2 times I have taken it for a drive. So I thought that maybe driving it hard somehow built extra pressure causing it to leak? I don't know if that is a valid cause, but I will check the radiator and hoses for leaks.
- I am not entirely sure of the point at where I shifted to be honest, but I was not doing super fast shifts either. The main thing about that is that it felt like while it was under 3000-3500 rpms it was acting sluggish and then it was like something kicked in and it got its power back.
- I thought the speed when I shifted to 5th and 6th gear might be too low. I have not driven it on the highway yet to test it at 65+ mph speeds.
I am not sure which software I will use once I get the cable, it is a USB cable so I will be using my laptop. I will probably use one of the ones listed on the Ebay ad.
You need to pressure test the cooling system for leaks. You can borrow/rent a tester in many auto parts shops. Pump it up to 18 PSI and see if the system holds pressure. The factory spec on the radiator cap is 18 PSI. The radiator pressure test device should also be able to test the cap.
When the pressure exceeds 18 PSI, the cap releases coolant into the reservoir to relieve the pressure. As the radiator cools down, a vacuum develops in the radiator and pulls coolant out of the reservoir to keep the radiator full. If there is a leak in the hose/metal pipe that connects the radiator fill neck to the reservoir under the battery, it can't pull the coolant back into the radiator, and now you have air in the system. If you are losing coolant, with the reservoir level dropping, there's either a leak or a head gasket problem. Every time you have to add coolant to fill the radiator to the fill neck, you have to bleed the air.
The best data logging software is Scan9495, written specifically for the 1993-1995 LT1 PCM. The author is a member here and can help with install and scanning issues. I review the data logs for members, and prefer a file from Scan9495. The data goes way beyond pulling codes, including ~50 sensor and PCM parameters that can help in diagnosing problems like you are experiencing. Free download, courtesy of GaryDoug.
One problem with the OBD-1 diagnostics is the fact they do not cover the high voltage side of the ignition system - cap/rotor, coil, wires, plugs. There is no misfire detection. That was added in 1996 and required the addition of a crankshaft position sensor. Tracing down ignition issues requires a lot of inspection.
Checking injectors can also be tricky. If the noid light flashed, you have 12 volts to the pink wire in each injector connector, and the PCM is supplying the ground requiredto fire the injector. There is a single code for electrical problems with the injector circuits, but it can't tell you which injector has the problem. Also no check on whether the injector is sticking, flowing too little or too much fuel. However the O2 sensor and long term fuel trim (LTFT) info in the data log can help.
I received the cable in the mail today and quickly went through installation and a brief scan of the car. Let me know if the attachment is what you need.
I only had it running for about 5-7 min and hit the scan button maybe about 2 minutes after I started it.
It shows to have 7 DTC's...yikes! (18, 23, 26, 41, 66, 69, 70)
As for the cooling system, the oil looks normal so I do not think it is a head gasket issue. I got a pressure tester kit from the local parts store and it started leaking at a little over 5 psi. I still need to lift the front of the car to figure out where the leak is coming from. Maybe I did not tighten a clamp sufficiently when I changed the Optispark.
I need a data log of the engine running under various operating conditions. This will be a .csv document with about 52 columns for various sensors and PCM parameters. The vertical axis is "time".... a separate liine of 52 data points for each of those columns. Typically a good log can take 15 minutes, and depending on the scan rate can produce 4,000-5,000 lines of data, containing about 250,000 pieces of data. I save this as an Excel spreadsheet, and look at it in detail.
@GaryDoug can help you produce the data log.
I've got a guide to how to run the engine during the scan.... I'll have to find the link but right now I have to sign off.
I shot a short video yesterday while I had the program running. The miss was pretty evident based on the sound of the exhaust when I accelerated it a bit. I had never heard it so clearly before. The car had only been running for about 3 min. Hopefully you guys can access the link.
It's really hard to try and detect issues by watching a video. Look how fast some of those numbers are changing? How are we supposed to read them? And, revving the engine with no load on it is virtually useless.
Starting with a still shot of the start of the video, there are obvious problems.
- IAT sensor appears not to be plugged in, wire is damaged, or sensor is faulty.
- is being prevented from going into closed loop by low coolant temp (119*F), and both fans are commanded on (they shouldn't be). May be in response to all the codes. The system goes into closed loopp after the O2 sensors read "ready" (yours are), a timer times out - typically 206 seconds, and your engine has been running for 279 seconds, and the coolant has to reach a variable number in the range of 120-140*F. Coolant temp is the issue.
- The Bank 1 (left/driver side) long term fuel trims are adding 25% extra fuel to correct for a perceived (could be real or false) lean condition. Bank 2 (passenger/right) is not having the same problem. Probably a bad injector, injector circuit or driver on Bank 1, hence DTC 18.
Fix the codes. Otherwise, you're just wasting your time.
18 - Verify key on/engine off/connector off injector you have 12 volts on the pink wire at each injector harness connector. Measure the resistance of each injector across the two injector pins. Start engine - .Use a noid light to verify the injector is receiving the ground from the PCM that fires the injector.
26 - make sure EVAP purge solenoid connector is in place. Check wires. Key on/engine off, check for 12 volts on the brown wire in the connector.
41 - Very hard to troubleshoot. Use the guide in the factory service manual, page 6E3-A-84. Most likely this is a "stored" code, ,because if it was an active code, the injectors would be shut down. Clear it (pull "PCM BAT" fuse (end of dash panel) for 30 seconds. See how fast the code comes back.
The video was only meant to show how bad the exhaust sounded but I am surprised to see everything you were able to determine from it.
I went out to check the IAT sensor and the wiring is damaged, so I will be looking at replacing that pigtail.
I did notice the difference in the fuel trims but was not sure how to read that. What you mentioned helps since it seems the issue might be on the driver's side. The only thing I noticed was the O2 sensor readings which did not seem to vary much for bank 2. It is my understanding that they should fluctuate between 0 and 1v, a little more similar to how the bank 1 sensor is behaving? I assume there is only one O2 sensor in the LT1 equivalent to a "sensor 1" (before the catalytic converter) on OBD2 vehicles? Would that mean the bank 2 O2 sennsor could be bad? Or is the extra fuel shown in the LT fuel trim the reason why the bank 1 sensor fluctuates?
I did the Noid light check a little less than 2 weeks ago and all of them would blink, but I will check the voltage on the pink wire, resistance between the two pins and follow your suggestions.
My plan is to find the coolant leak first and take care of that. I will post back the results of my voltage tests once I get to them.
This is all very helpful. Thanks again.
I was able to get the leak fixed. It was just a hose clamp that was not fully tight after I changed the optispark.
I also decided to change the fuel filter. The old one I took off made a rattling noise so maybe that is what caused what felt like a miss or lack of power originally.
I also changed the EVAP purge solenoid valve and repaired the wire going to the IAT plug; and the Service Engine Soon light went away.
The engine developed a bad oil leak while doing that first test drive (burning oil on the passenger side exhaust with some smoke coming through the vents) so had to trace that and apparently it was coming from the rear of the passenger side valve cover. I removed the valve cover and gasket, which looked to be in good condition and just added some adhesive to the valve cover when reinstalling.
When taking it for a second test drive, the SES light came on for like 30 seconds, then turned off. That happened about 3 times. I am guessing it might be related to the ICM code. I did not hook up my laptop to check if the code stayed in the memory.
The temperature was high though. It went up to the "3/4" mark and stayed there for the most part during the 2-3 mile test drive. Looks like activating the AC helped it go down (when idling in my driveway) so maybe it is related to the fans? I drained the radiator when I fixed that bottom hose clamp, so maybe I got air in the system although I did see a steady stream coming out of the bleeder screw before closing it. The lack of power seemed to have gone away on that second test drive, but since the temp was high I did not want to drive it for too long or too hard.
Do you have a code reader to tell you what codes are being thrown? There's not one but actually two bleeder screws in the engine bay, make sure to use them in the correct order and with the correct bleeding procedure. Was the check engine light coming on or actually flashing when you were driving?
I do have a code reader. See previous posts with the DTC codes I got when I first used it.
All the ones related to the AC went away on their own as well as the injector related one. I had previously found that the connectors for injectors 6 and 8 were swapped. Maybe that was what triggered that code?
One of the two wires going to the IAT plug was cut, so I fixed that and it took care of code 23.
Replacing the EVAP purge solenoid fixed code 18.
The ICM code seems to be intermittent. I replaced the ICM along with the coil when I changed the Opti.
I followed the procedure in the link. I had only drained the radiator so it only took a little over a gallon to fill up. I will check it again today.
I do have a code reader. See previous posts with the DTC codes I got when I first used it.
All the ones related to the AC went away on their own as well as the injector related one. I had previously found that the connectors for injectors 6 and 8 were swapped. Maybe that was what triggered that code?
One of the two wires going to the IAT plug was cut, so I fixed that and it took care of code 23.
Replacing the EVAP purge solenoid fixed code 18.
The ICM code seems to be intermittent. I replaced the ICM along with the coil when I changed the Opti.
I followed the procedure in the link. I had only drained the radiator so it only took a little over a gallon to fill up. I will check it again today.
Ok Gotcha, I see the code you're getting now. You may want to verify that the code coming up while you are driving is actually the ICM Code and not something else. You can plug in your reader, let it set on your lap and once the CEL comes on you can 'read' the codes. Glad you were able to fix all the other issues successfully, that's definitely a big step in the right direction. Filling the cooling system on these cars is a bit tricky than other cars but not hard by any means.
All the ones related to the AC went away on their own as well as the injector related one. I had previously found that the connectors for injectors 6 and 8 were swapped. Maybe that was what triggered that code?
Replacing the EVAP purge solenoid fixed code 18.
DTC 18 is for the injector circuits, not the EVAP purge solenoid. The fact that the wires for #6 and #8 were switched would not set that code. The PCM has no way to know what wire is on what injector.
DTC 18 is for the injector circuits, not the EVAP purge solenoid. The fact that the wires for #6 and #8 were switched would not set that code. The PCM has no way to know what wire is on what injector.
Sorry about that. I meant "Replacing the EVAP purge solenoid fixed code 26."
Code 18 did not come back after that original first scan. Same for the HVAC codes.
07-15-2020, 10:57 PM
