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Purchased this 88 4+3 car off ebay two weeks ago, 49K miles, great paint, nice interior. It arrived late last week and we were barely able to keep it running to get it off the trailer. Once it warms up it seems fine, it might have a bit of a stumble when the RPMs drop but I've not quite nailed that down yet. I of course did the rookie mistake of throwing a few parts at it... new TPS (calibrated to .54 volts) and IAC valve but no dice. Seller did include all the docs from the previous owner including a Service Manual and a ton of receipts. In the manual the page on Rough idle is dog-eared and a page of notes on what he tried. I think the original owner that did all the mods (circa early 2000's) struggled with this issue from the get go, sold it to the guy I purchased it from a couple of months ago. He probably realized he was in over his head and sold it via eBay (without disclosing the idle issue). To his credit he has offered to help if it gets costly to fix. I don't see an EGR valve on this setup so I kind of ruled that out. Also the AC is out but I knew that before buying, I mention that because I read it might effect cold idle?
Any and all help solving this mystery would be much appreciated!
Mods are:
TPIS/AFS Heads - Unable to determine specs yet
Mini Ram manifold and fuel rail kit
ZZ9 Camshaft
Long tube headers
170 degree thermostat
Level VI PROM, Mini-Ram, 86-89 F&Y (this is the updated PROM maybe?)
Attempting to attach video of cold idle
Last edited by TA76; Aug 3, 2021 at 10:49 AM.
Reason: attaching video
Eh, can't get the video to work. It surges to 16-1700 RPM then sputters down to 3-400 if it doesn't die it surges again. It does this until Temp reaches 170 or so.
Try unplugging the MAF and see how it runs. If it by chance runs better then you have a MAF sensor disconnected that indicates the MAF needs to be cleaned or replaced.
Check and see if the Coolant Temperature Sensor is saying when the engine is cold.
What happens after 170 or so degrees? Does it get smoother or rougher?
There are several resident experts on the the TPIS systems and equipment. Hopefully one of them will jump in here and help you with more detailed information.
The Engine is supposed to start up in "Open Loop" and when doing this it uses the temperature data and runs off stored data. Once the Oxygen sensor comes on-line then the engine should transition from Open Loop to Closed Loop. Is your Corvette doing anything like Open Loop Transitioning to Closed loop or is that what happens around 170*?
Thanks, I have unplugged the MAF, no difference. I have not checked the CTS but will do so. The CTS is a separate sensor than the one for the temperature guage right? Yes, at 170 the idle settles down to around 800 with no surging or sputtering. The car seems pretty driveable at that point but to be honest I've only put about a dozen miles on it since it arrived. I'd really like to get the idle sorted out and the tires are circa 2005 so waiting on new ones to come in (on back order like everything now!).
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Since this car has traded hands a few times.i doubt it's simple. IDK what a level xx f & y prom is but it sounds generic. The problem is probably the tune. Since it's loaded with TPIS parts, I would contact them for some direction. You need to.make datalogs prior to calling so you can answer questions. You should also check fuel pressure, If ECM is expecting cold start injector operation; engine would be lean because it's not there.
Thanks guys! What do I need in order to do data logging? I do lean to this being a software problem. I'm going to try and contact the owner that did the mods and see what info I can learn.
Edit - Found this which seems to offer a wealth of info: http://obd1.com/
More selections that I would have thought for software choices, any recommendations?
Last edited by TA76; Aug 4, 2021 at 09:18 AM.
Reason: add link
Bet its an old TPIS chip in it.
If you have no vaccum leaks, fuel pressure/injectors check out Id call pcmforless like 383vette said. Bet they have a good tune on file for your mods those were common back then. They offer a retune also, for the money you cant beat it.
Bet a good tune (chip) fixes your issue. Make sure all else is working right first
I have some tuning stuff here have to figure out what it all is...cables discs etc Id offer cheap
Unless you know tuning Id pay someone else its too cheap not to.
Soon after start up, the surge began. I blipped the throttle a few times then hit my (proprietary) idle launch assist (A/C) button (+1800 rpm) and switched it off again to force the IAC into action. The surge decayed as the revs dropped following to normal desired idle (725 rpm).
In this case, the initial surge was excited by the long term fuel trim (BLM) toggling between 118 and 122 as the cell boundaries (rpm and airflow) were crossed, causing the rpm, spark advance and injector pw all to vary and further amplify the surge. Once this pattern occurs, the ecm can't really do much to stop it. In neutral, only the rotating mass and engine friction is there to provide any rpm stability.
However, once the BLM and BLM cell toggling stopped due to the rpm/airflow behavior, the surge quickly decayed on its own and normal idle resumed for the duration as can be seen.
How did this happen and why? The ecm retains 2 BLM values in stay alive memory (SAM) from the preceding ignition cycle/drive (temperature dependent). The cell 0 blm value is stored into SAM cell A, and the cell 10 blm is stored into SAM cell B. On the next start up, cell 0 is set to the SAM cell A value (118 in this case), and all other cells (1-15) are set to the SAM cell B value (122 in this case).
Worth noting is that on the previous drive, I had a clutch failure. One of the slave cylinder bolts had backed out allowing the slave cylinder to pivot on the remaining bolt instead or disengaging the clutch. I had to start the car in gear (like a jeep), synchronize my shifts and nurse it home with an engaged clutch. Not fun. This lead to unusual speed/load/rpm/airflow conditions (mostly low speed/light load in 1st gear) that lead to screwy/unusual blm values being stored into the SAM cells.
This was not really a tuning issue, but a conditional issue that was influenced by the tuning, control system design, and resulting behavior.
Long story short: surge events can be complicated and be caused by any number of disturbances, tuning, internal, external or otherwise. Without data and analysis you will always be chasing your tail to find and correct the cause. In your case, I would bet that the spark advance table and idle rpm table need some work. Who knows what else could be wrong.
As part of this exercise, I've gone ahead and revised my own spark advance and idle rpm tables and increased the SAM cell lower limits from 118 to 124 in order to make it less susceptible to surge under these conditions:
I spoke with the guy at pcmsforless, after discussing the mods (including camshaft specs) he seemed pretty confident that he could fix my issue. I have located several shops in my metro area that I feel are capable of tracking down the issue. The way I see it I have three choices:
1-Order the new tuned chip from pcmsforless $400 and hope that does the trick.
2-Go with a local dynotune shop, was quoted $350 for one but I'm not sure what all that includes so the price for putting this in a shop is somewhat open ended.
3-Get the equipment needed to do some datalogging, post it and hope one of you guys would be able to help me figure out what is going on?
After looking thru everything the original owner did I'm really thinking this is a software issue.
A couple more log examples with the aforementioned changes. (Note: 2 csv files are included in one link, can display either file with the change log button):
Nothing too exciting here, different conditions including start up and running with warm coolant temperature, but at least no surging this time around. Lost data connection early in first log, so a couple events are shown including some dead time. I"ll see what happens on the next cold start, but should be improved.
Last edited by tequilaboy; Aug 4, 2021 at 01:19 PM.
It can be annoying, but is not always avoidable despite best efforts.
Other potential sources of error could include injector linearity issues and/or maf calibration issues. Keep in mind I'm running 63 lb/hr injectors (in double fire mode) which aren't all that great at maintaining linearity at these very low pulse widths (down around 1 ms and below). Comp 503 cam is also a contributing factor with 224/230 duration and 112 LSA resulting in significant torque variation with small changes in rpm. I tend to run the lowest possible idle rpm (within reason) in order to maintain a clean idle with the large injectors and no cats. Sometimes it is what it is.
Last edited by tequilaboy; Aug 5, 2021 at 12:16 AM.
Finally got a datalog using TunerPro RT with an ADX file for a 88 L98... setup a Datazap account but I can't seem to get the chart to work like yours does tequilaboy. Unfortunately the link to the video tutorial shows "unavailable". I ran the car this morning from a cold startup. It ran for a few minutes, surging, sputtering and then died (I usually sit in the car and blip the throttle when it starts to die, didn't today). Hoping at least the raw data might tell me something but being a complete noob to this I have no idea what I'm looking at. Hoping someone can give this a look for me?
You need to edit/clean up the csv file a bit first. I typically copy then delete the first row with the tunerpro log date stamp info, paste this into the comments and then delete the row with the unit labels, and realign the data leaving just the column headings in the first row and the data in the subsequent rows.
I have a similar issue on cold mornings, my engine does not like to idle when its cold in the garage at all, it'll cut off if i don't hold it up just over 1000rpm til it smooths itself out. Warm days, hot days, she cranks and idles fine.
tequilaboy has you on the right track, you must log and look into it. It can be tweaked in the tune. Don't throw parts.
OK back, this looks better! O2 sensor looks wonky to me, RPM jump corresponds to O2 spikes or is it the opposite? Also noticed near the end of the log, everything seems to smooth out which is what I observed watching the car idle. It actually seems to be idling somewhat normal, then it just flat dies. Looking at the chart during that time frame, the airflow keeps dropping and dropping. Don't know what all this means but hopefully one of you guys do.
That is why someone asked above if unplugging the MAF alleviated it. If you are not getting MAF codes set, I would hold off before going through the manual's diagnostic.
Stick down the tuning path, the datalog will show if something is up with the MAF.
In your data log i did not see engine timing, this will be critical to solve this problem, can you get a data log showing the timing values?
It would also be very nice to have your AFR to determine if you are rich or lean. A cold engine wants a rich AFR and a steady AFR, if this is moving around it will effect the idle.
My guess is you have changing timing table and an moving AFR (not steady) while the car is in open loop and warming up. The fact that the idle steadies out when the car hits closed loop and warms up makes me believe this is more related to AFR but timing also is a big player.
In some cars I need to drop idle timing way down so the engine cannot accelerate at low RPM so quickly, which allows the IAC to respond fast enough to keep a steady idle.
Heavly modified 88 cold idle issues
