KyleF

TPI - L98 - 88 - 700R4 Trans. It will Idle in P or N normally, drop in in R or D and all but dies. It will kind of idle around 300-400 rpms and lug around, but the slightest throttle input makes it want to die. While it is doing this, a shift back into N and it will rev. freely. After I let it run this way around the neighborhood once or twice, it starts to run normally and then eventually everything seems normal. No Codes at any time.

I can find thousands of "runs fine cold but nor warm" threads, but haven't found any doing the opposite.

I have Cleaned the passages and replaced the IAC as a maintenance item last fall.
Had a leaky fuel rail O-ring that I repaired last sumer
Did the IAC reset procedure when I replaced the IAC... when warm it has a good idle around 500-600 RPMs in D and 700-800 in P.


Going to get some scans and check fuel pressure(s), then pull plugs and have a look... but I really can't find info about what could be causing this. Even on the general internet, there seems to be plenty of runs good cold and not warm information, but not the opposite.

Kevova

Does coolant sensor read ambient temp prior to cranking engine. Vacuum leak cold manifold seals as temp goes up. Low fuel pressure disconnect and plug FPR (make sure no fuel) see it increased fuel pressure helps. 88 was last year for cold start injector if it not working another possible cause.

ctmccloskey

Like Kevova above I have a feeling that it is likely caused by "bad information" getting to the ECM.

My first guess would be to verify the Coolant Temperature Sensor is accurate.

What about the Throttle Position Sensor? Did you verify it is set properly? I would double check it if you could.

If the 9th injector was operating normally the car should be idling faster like 1000-1200 rpm. If the 9th injector was being "called for" and was "not working" that might produce the symptoms that you are experiencing. A potential scenario could be that the Car is wanting the 9th injector to work and it is not functioning and once the engine warms up completely the problem is gone as the engine does not need any more fuel enrichment. This again would point to erroneous temperature sensor readings potentially.

Low Fuel pressure would also cause the idle to be that low, I would again follow Kevova's advice and check the pressure and verify that your Fuel pressure regulator is working properly.

Since it happens when cold you might go to an auto supply store and get a can of "Freeze-It" (can of spray that Cools it down to really cold) and try that on the components that are suspect. It will make the job easier.

How many miles are on the Corvette? Do you have any kind of scanner available to monitor the engine as it goes through the startup process?

The last time the car was operating normally was before you installed the new Idle Air Control Valve? Did you replace it with the same style pintle on the top of the IAC that goes into the throttle body and controls the air to make the car idle properly. I ordered a replacement for my 1988 C4 with the 700R4 and when I compared it with what came on my car it came with a different shaped pintle that would NOT work with my throttle body. If you installed the wrong shaped IAC then you have your answer there probably.

The number of miles is important because throttle bodies do require maintenance regularly and have been known to leak a bunch of air around the shafts of the throttle body introducing more air than the car is prepared for. But since your problems seem to be related to the temperature I would go to the CTA first and verify it is still working accurately. Do you have the Factory Service manuals for the 1988 model year? There is a chart that shows the resistance value and the matching temperature which allows you to verify your sensor is accurate enough.

I have a 1988 C4 Coupe with the automatic and I have had my fair share of problems. I just bought a Snap On scanner to use for working on my 1988 C4 and look forward to having all that information in front of you easily. I had a problem where my fuel supply line rusted shut internally over time, that was a real PIA to figure out. I had to measure the fuel flow rate to find the problem, not an easy problem to figure out.

Let us know what you find when you do get this problem figured out. I have never seen this issue before and would really like to help solve it and learn from it!

Best Regards,
Chris

KyleF

It's actually my IROC, but being that TPI/L98 applies to both I thought I would ask here. It has 62K Miles.

My C4 Vette has an LT4.

I am using a laptop, Moates.net cable w/10K Ohm resistor switch, and WinALDL.

It was my first guess too. Seems like everything else is right and has been remedied and except for this brief time all runs excellent.

CTS was showing appropriate readings. When I started it was showing 107° and warmed up as I let the car run. I had been driving it earlier in the morning (Cars & Coffee) so I feel this is appropriate even if not 100% spot on. The intake and all was still very warm to the touch. At no time was the information at the ECM way out of line.

TPS Volts was .53V at idle and 4.63V at WOT (Using pedal and WinALDL) so this is what the ECM is actually reading.

KOEO: 42PSI
Idle (W/Vacuum): 38PSI
Idle (W/O Vacuum): 46PSI

Leak Down
0 min: 42PSI
5min: 34PSI
10min: 25PSI
15min: 22PSI
20min: 19PSI
25min: 18PSI
30min: 16PSI


I don't know I did replace it. I don't remember it looking differently, but I don't remember really looking for that either.
I have owned the car for about a year now. It never had a idle that I thought was great but it has gotten worse. Originally, it just stumbled off idle when cold and would always run great over 1500 RPMs, the idle issue has gotten worse. This was what originally prompted doing the basic set timing, adjust throttle position, re-set timing, confirm RPM and IAC counts, adjust TPS (IAC reset Procedure).

As mentioned, 62K. If it is worn out, it would be the only thing on the car. It has really been well cared for and not abused. The Previous Owner sold it as it was doing more sitting than anything else and he didn't want to see it rot. So, I got it with a few minor issues, but was able to hop in it and drive 1.5hours the day I bought it with no issues.

No I don't. Never needed them. Between here and another forum dedicated to F-Bodies I can usually find any information I need and more than what the FSM tells you TBH. Though WinALDL does give you both raw and coverted data. So, I can see exactly what the ECM sees.

I could also check this, but II think if it can run hard under boost at 4K Rpms it should have plenty of volume to idle.


I am not sure what I consider suspect at this point. I was betting on CTS or TPS doing something odd. Might have to revisit this idea.

You might be onto something here. Last summer the car sprung a leak on the fuel rail to the 9th injector. This rendered the car unable to run since it couldn't really get fuel. It was dumping pressure from the entire rail. When it is idling well, the IAC counts are low (20 or so) at 1K rpms or so, but when it goes into this funk at 400-500RPMs, the IAC is at 160.

If this is the case, all is working, the IAC is doing what the ECM is telling to to do, but all the air in the world isn't going to fix a lack of fuel situation. However, when it is 60° outside I would think the cold start injector is about meaningless. Also, doesn't lean usually cause a high idle? Nothing wrong with confirming it's operation. What is the best method to test?


Yup, I can find 100s of threads where cars idle in open loop cold very well and then go to crap when they get warm and go into closed loop. I have found 0 threads related to this. I usually read before posting. I hope to learn from this too and I appreciate the ideas.

What I have now is:
IAC Could be the wrong style
Throttle Body Leaking when Cold
9th injector not firing
Fuel Flow Volume.

Keep the ideas coming.

KyleF

CTS was showing appropriate readings. When I started it was showing 107° and warmed up as I let the car run. I had been driving it earlier in the morning (Cars & Coffee) so I feel this is appropriate even if not 100% spot on. The intake and all was still very warm to the touch. At no time was the information at the ECM way out of line (Warm or Cold).


Doesn't this actually make an engine idle high? The IAC is going high when it's stumbling, calling for more air. Would that make sense if there was a leak? When Idle is solid it is pulling 18"Hg. Of course this drops to 10 or so when it starts to struggle.

KOEO: 42PSI
Idle (W/Vacuum): 38PSI
Idle (W/O Vacuum): 46PSI

Leak Down
0 min: 42PSI
5min: 34PSI
10min: 25PSI
15min: 22PSI
20min: 19PSI
25min: 18PSI
30min: 16PSI


Certainly something that needs to be verified.

Keep the ideas coming.

ctmccloskey

There is one book you might want to add to your collection.

Author: Charles D. Probst
Publisher: Bentley Publishers
Title: How to understand, Service and Modify "Corvette Fuel Injection and Electronic Engine Management"

This book really goes into a lot of detail explaining HOW things work the way they do. I have read it and use it for reference regularly. It is well worth the money in Addition to your Corvette's Year in the Factory Service Manuals. The most important books are the Factory Service Manual set for the appropriate year Corvette. Please don't try Clymers or Chiltons as they are not detailed enough.

Have you checked the back pressure from your catalytic converters? I had a VW Scirocco and one day I tried starting it and it would idle fine. It would not rev up past 1500 rpm and upon opening the hood I found out why. Every time I hit the gas a cap on my Emissions sniffer hole would inflate like a large balloon and when you let off it went back to normal size. I suddenly realized that I had a plugged catalytic Converter so I removed the cap and drove it to a gas station at 15 mph. The mechanic removed the catalytic converter and I took it out back and removed the blockage with a large flat blade screwdriver. After shaking all the pieces out I had it re-installed and I drove it 300 miles home that day. Running too long in "open loop" is not good for your catalytic converters. Just an idea.

In the Book mentioned above there is a paragraph that I will copy for you.

"The Oxygen sensor inputs a signal to the ECM which OUTPUTS to the Fuel Injectors. The signal tells "how much" fuel is needed to control the air/fuel ratio. Each fuel injector receives a pulse signal according to the continually changing oxygen sensor voltage signals."

"The oxygen sensors and distributor references are the Primary inputs. Secondary inputs include sensors (IAT, ECT, TPS, MAF/MAP) that advise the computer to enter or remain in Closed Loop or switch to Open Loop. While reading from this book I learned another interesting factoid about the L98. "During Crank, don't step on the accelerator even a "Little". Above a 3%TPS signal the computer would increase the Pulse time and perhaps flood the engine. Of course Pumping the gas pedal is a No-No as the TPS signals really upset the ECM."

I am not sure of the age of your O2 sensors but when I was younger I was taught to change the O2 every 50,000 miles. As they age they slow down their response time and can really cause strange problems. Verify that your are working all the time in closed loop if possible. I don't have anything as elaborate as your software and cables but monitor the car as it transitions from open to closed loop if you can. It could be something as simple as a fading O2 sensor that is lagging to far behind for the speed of the ECM. It might be something to do with your MAF sensor as well, have you verified the output? Is there any air leaking around your MAF sensor? A faulty MAF sensor will clearly mess your ECM up and keep it running poorly, the wires in the MAF are so fine it doesn't take much to screw them up, they are 1/10 of a Millimeter in diameter and fragile.

I am going to continue reading what this book has about the problem you are having. Clearly one of the secondary sensors is not working 100% of the time. Could it be the distributor reference signal it sends to the ECM?

I will keep you posted on any new ideas!

WE WILL FIGURE THIS OUT!

Best Regards,
Chris

Check that MAF out well, if it can draw air around it the sensor is useless. Your mention on the Idle Air Control valve goes "High" when stumbling sounds like it is getting wacky airflow readings which again would point to the MAF in your intake.

KyleF

I find the Chilton and Haynes as a good quick reference. I agree though, not very detailed. I do have the proper ones for my 96 Vette.

Though, this is an IROC

What Catalytic Converter? Seriously, PO had it removed. Headers to true Y-Pipe. 3" pipe over the Axle to a Borla Muffler.

I have been through the clogged Catalytic Converter thing with a 5.0 mustang. They would glow red LOL.


Replaced it last summer, or about 800 or so miles ago. Mainly doing some general maintenance and some was because of the idle issue. Its a factory replacement Bosch and it is working. I can see the voltage and cross counts in the scanner. The PO installed a gage to read the Wide Band... and it illuminates and puts on a nice light show. So, the ecm is getting readings.

I wish. Car runs great in closed loop. It has to be warm to get there. Once it is warm it is 100% good to go. Or well, seems that way. The issue here happens about 10-20 seconds after start and continues to be an issue until it is warm. Possibly is an issue until it gets into closed loop.

I greatly appreciate the ideas.


I was laying in bed last night looking at ICMs... I am not sure if it sends the reference signal or not. I really hate throwing parts at a car to solve problems without something pointing to it being bad. I have no issues on the tach or any reason to believe the RPM signal getting back to the ECM shows anything other that accurate RPMs. As much as I changed the O2 for general maintenance, I have not done anything to the ignition.

I was thinking of changing the plugs, wires, cap, and coil... I have no idea how old they are but they can't have too many miles on them. The Ignition is composed of a MSD6AL Box, MSD boost Retard Box, MSD Coil, MSD Cap, (I would assume rotor), and wires. None of it is factory, so I can't image any of it having too many miles on it.


I need to check it out. Another person mentioned something may be in front of it on the screens or something. So, it is on the list. It is a reconditioned Bosch Hot Wire style MAF. I had to replace it from the Spectre that was in there befopre that kept causing a Code 36. No more 36!

Ah, Fun little cars.

KyleF

So... Last night it acted a little differently. Possibly because it was in ALDL mode to grab the data.

Here is a log from Start Up attached to the post.

Here is a video of it idling in ALDL mode.

KyleF

New development. I noticed while I was doing scans, Code 36 flag was still on. I hadn't reset the computer since I swapped MAFs. So, I disconnected the battery, hooked it back up, and not it will barely start. Will only stay running if I work the throttle.

And I mean work it. I have to apply throttle and let go. When opening the throttle, I can get the RPMs to come up and prevent it from stalling. However, if I leave it down (Say about 1/4 throttle and about 1.2V on the TPS, it will stall and die. If I don't do anything it will stall and die, but if I work the throttle in and out all is well.

Kevova

The supercharger is a major detail left out of intial posts. Adjustment to tune may be required. Has it ever run correctly cold with supercharger?

KyleF

Yes, it has ran much better before, but has never been "right" since I owned it.

The supercharger is not doing anything at idle. This is an old school system and only running 6PSI when it is at WOT and higher RPM. The system has a boost retard box instead of a custom tune, FMU, a second pump, and is MAF. It is a bolt on kit, not a tuner kit. It has a bypass valve that is operated off engine vacuum so it just recycles the air until the car has significant throttle and load. Old School.

Would it benefit from a tune? I would imagine there are a few HP that could be found but, that would mainly focus on when the car was under throttle. Not idle. Which, when it was able to run well enough to get warm - Once warm everything worked great.

This is a MAF system. It is pretty simple... g/s flow with a given throttle yields timing and fueling. To compensate for the boost the added fuel pressure provides more fuel at the same pulse width and the Boost box pull a degree of timing for each pound of boost.

In the absence of boost, everything should be acting as if the Supercharger isn't there as it should because the supercharger isn't effecting anything at idle.

Kevova

IDK. Tuning comes into play from start up. I would try to contact compressor dealer/ installer for information. If you have adjustable fuel pressure regulator try raising fuel pressure to 50-55. ECM is running preprogrammed tune until it goes into to closed loop. Close loop requires active O2 and temp increase. I think you will need a experienced tuner who's familiar with what you have. Tuner will likely need to physically inspect car as part of the tuning process.

KyleF

I appreciate the help, but please don't steer it this way. The Supercharger is not adding any extra air in at idle. Engine vacuum and MAF flow is exactly the same as a stock motor. When purchased, I turned the key and drove home for over an hour. It had a slight stumble off idle but always ran great over 1500RPM. It had the exact system it has now.

This is not a tuner kit. This is a bolt on kit. Well proven and been available for 20+ years and a new tune (chip in this case) was never provided with the kit.

tequilaboy

The posted data doesn't really fit the 87-88 $32B 160 baud datastream, so some values appear to have strange readings.

;-------------------------------
; ALCL Xmit Table (160 BAUD)
;
; Table Of Addr's vs DATA
;
; ECM $32B
;------------------------------
ORG $0713

FCB $C000 ; 1, PROM ID LSB
FDB $C001 ; 2, PROM ID MSB ;
FDB $002C ; 3, IAC Present Posit.
FDB $005D ; 4, Coolant temp, (A/D)
FDB $0065 ; 5, Filtered MPH
FDB $0112 ; 6, EGR D/C
FDB $0057 ; 7, RPM
FDB $0081 ; 8, TPS (A/D)
FDB $00C6 ; 9, Base Pulse Closed loop correction (CORRCL)
FDB $006F ; 10, Filtered o2 Sig
FDB $0005 ; 11, Err Flg #1
FDB $0006 ; 12, Err Flg #2
FDB $0007 ; 13, Err Flg #3
FDB $0008 ; 14, Err Flg #4
FDB $0044 ; 15, MWFA1
FDB $0060 ; 16, MAT
FDB $0037 ; 17, MCUINST
FDB $00A1 ; 18, OLDPA3
FDB $00C0 ; 19, BLM
FDB $00F1 ; 20, ALDL Counter
FDB $00EA ; 21, MAF display flow Gms/Sec
FDB $00EB ; 22, LSB
FDB $00D5 ; 23, Base PW (Last Inj), 16 bits
FDB $00D6 ; 24, LSB

Please do yourself a favor and get proper logging software that will work at 8192 baud.

Note: The first byte of the datastream (MW2) is not shown on this list since it is defined at another address deep in the code. This list is really bytes 2-25.
MW2 is always the first byte in both 160 baud streams (5 bytes sent to dash and the 25 byte ALDL mode datastream).

KyleF

Well, it's obvious you have a lot more experience with the electrical side than I do. I am using WinALDL. It has allowed me, on this car and others, to see what I need to see as far as sensor values. Rather than probing with a Multimeter, plug in and read. The logs are just a snap shot of each frame that is generated. Roughly every 1.5 seconds.

So, what values look odd?

What software do you recommend?

tequilaboy

TPS looks strange (2.6) INT (should be CORRCL), BLM, OLDPA3 (knock count), and rich/lean count are obviously wrong/invalid or out of order in the stream compared to what WinALDL is expecting.

Also, bin appears to be 1988 ABYA, based upon prom id.
ABYA 5400 8461 Auto 2.77 Fed F
ABYA 1988 31916 16075399 1227165 5400 8461 FB L98 5.7 MD8 AUTO GH3

I like to use TunerPro/TunerProRT for tuning and scanning due to its flexibility, but it requires some user involvement/knowledge to properly setup adx files. DataCat works well and is easy to use. Other scanning options include EFILiveV4 and Datamaster if you can find.

KyleF

I noticed the TPS as well, but I think that is only showing during cranking. in a KOEO or idling situation, the Voltage was spot on in the sweep.

BLMs.. I notice they are going high, but so is the IAC.

What is the .bin for? The car has a swapped rear end in it. I would have to look for RPO codes to see if it was originally a 2.77 car or not.

KyleF

Will my Moates Extreme ALDL Cable work with the other software at the higher rate? I don't care to pay for a better software, I just want to know it works. I saw this on tuner cat's site. Why is the 87-88 5.0 Supported byt not the 5.7? Shows compatibility with the Corvette L98 of those years.

F & B/D-Body Cars
87-88 F-Body 5.0L PFI (LB9) 8192 baud
89 F-Body 5.0L PFI (LB9)
90-92 F-Body 5.0L PFI (LB9)
90-92 F-Body 5.7L PFI (L98)
93 F-Body 5.7L MFI (LT1)
93 F-Body 3.4L V6 (L32)
94-95 F-Body 5.7L (LT1)
94-95 B/D-Body 5.7L (LT1)
94-95 B/D-Body 4.3L (L99)
94-95 F-Body 3.4L V6 (L32)

Y-Body Cars



87-88 Y-Body 5.7L PFI (L98) 8192 baud
89 Y-Body 5.7L PFI (L98)
90-91 Y-Body 5.7L PFI (L98)
90-92 Y-Body 5.7L MFI (ZR1)
93-95 Y-Body 5.7L (ZR1)
92-93 Y-Body 5.7L MFI (LT1)
94-95 Y-Body 5.7L (LT1)

KyleF

To post an update: The end result was the "new" MAF was bad. Didn't get the Code 36, but ran like crap. Back to the thin film MAF, car runs much better, but Code 36.

I have set up Tuner Pro RT and have verified the car was a 2.77 rear.

Unfortunately, only able to get data in the ALDL mode. How do I upload a .XDL file for review?

ctmccloskey

That is great that you were able to get that sorted out! I have heard of way too many "New" MAF sensors that were defective from the factory.
That would explain a lot of the symptoms you were experiencing.

How goes the progress of learning to communicate with your car? I am getting ready to order a Moates cable to use on my L98 setup. I am leaning towards Tuner Pro so let me know what you think of it.

This engine issue of yours had me stumped for a while but a defective "New" part will mess you up in a hurry! I reread sections looking for an answer and continue to learn every day. I suspected that one of the main components required for Closed Loop was not supplying accurate data, thank goodness the list is fairly short in the Closed Loop mode!

Thank you so very much for enlightening us with what the issue was with your L98. This is one of the many benefits here at the Corvette Forum as we all learn when we share the problems and then discuss them with other people involved in our hobby!

Good Luck KyleF!

Best regards,
Chris