Problems in TPI Build
Thread Starter
7th Gear
Joined: Oct 2019
Posts: 7
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Problems in TPI Build
I have rebuilt a 79' Chevy K10 and decided to a bit of modernizing of the drive train for fun and for drive-ability. I have had a problem with a surging idle, hard cold starts and poor performance (mpg and hp) on the freeway in my top gear. I will post the build of the drive train, the problems in more detail, and what I have done to investigate possible problems. If it is the nature of the build and setup of the truck I guess I can live with it for now and change anything in the future, but if you have something I should try or look for; I am all ears.
Surging Idle:
The truck when in gear or not in gear will surge up and down 200 RPMs when the car is warming up then for the most part will settle when the truck hits full operating temperature. Occasionally, the O2 sensor will show a sudden drop to 4mV that lasts a second or so. This seems to correlate to the moment the RPM's drop causing the ECM to counter the drop and spikes the RPMs.
I have so far tried to mess with ECMs VE tables to see if there are any unevenness in the Idle areas and it doesn't seem to change where and when the sudden drop of voltage happens in the O2. I also raised the idle rpms from 650 to 700 and found a very slight improvement. I changed the ECMs IAC- stall saver rpms, SA idle high and low parameters and found it to improve the severity of the surging idle.
Hard Cold Starts:
To be honest I am not entirely sure what is going on here, the truck will crank and catch fast when its cold and will run for about a second then just shut off, it will do that 3 times if i don't touch the gas pedal before it can run by its self. Or will stay running the first time if I put just a little pedal in; it will run for a second or two with that slight pedal and then Idle by its self from then on. Another weird thing is that if I start it and get it past the hard couple seconds it can restart just fine then on, even while still being cold. Hot or mildly warm restarts are perfectly fine as far as I can tell.
I have tried to mess with the spark advance added when cranking and it seemed to make it idle better or be less likely to shutoff on me. I have messed with the IAC positions parameters and it just made it worse by increasing the time I have to press the pedal in before it could idle by its self. I have also tried to restrict the PCV flow as that was drawing 5 in of vacuum in the case. It is now pulling 1 in of vacuum. I don't see smoke at startup or during idle.
Poor Highway Performance:
To best frame this problem I am going to go through the 3 stages of builds in this truck. When I first bought the truck it had the same engine I have now and had been rebuilt by the previous owner with an engine swap a 71' 4- bolt main SBC bored .03 over with flat tops. I am not sure of the exact piston to deck clearance but it was really close to the top of the bore. It had the stock 75cc cast iron heads, and a 268H cam and an edelbrock intake and carb. It ran fine but seemed to lack in drivibility and was leaking from a couple of gaskets, so I tore it down for the second build. I put new gaskets and seals on the entire engine and fixed a cracked exhaust valve, dropped a 700r4 in and converted the intake to a TBI unit. Ran great for a while but junkyard electronics for the TBI kept failing and it was lacking in performance on the freeway, so I decided to do a ground up restoration on the truck and do a lot more swapping of parts (I will list below). The truck runs about the same as it did with the TBI unit, but now gets about 3 mpg less than before and like before still looses a serious amount of power if I have to climb a small incline at freeway speeds 70 mph at 1700 rpms and won't respond till I force it to down shift and/or place it a WOT. Around town in runs strong and seems to have a lot of power and gets better mpgs than on the freeway.
I have tried to remedy this by closing down the PCV as it was sucking too much and tried to adjust the timing tables. So this is where I am stumped the most. I can increase the timing up to 56 degrees advance almost everywhere on the table and the truck won't ping, detonate, or change the characteristics of, well anything. I have adjusted the base timing on the distributor so I can advance the timing higher before it jumps terminals. The only time the engine cares about timing is at WOT. I can't advance it above 34 degrees or it detonates a lot. If I drop the timing too far the engine will get more sluggish in any of the areas on the spark table.
Some random facts about the truck that might be relevant. I found the PCV was sucking in oil and was depositing carbon on the valve side of the spark plug, but was burning fine on the piston side; fixed that the other day and doesn't seem to change anything. I am at around 5,000 ft. It cruises down the highway at 70 kPa despite what timing it is at. Idles at about 40-45 kPa. 18 mpg around town and 15 mpg on the freeway. VE learns go in circles depending on the day. Cold days typically removes fuel, warm days it adds fuel.
Drive Train Build Specs:
700r4 with stock converter and corvette servo.
3.73 gears with 34 in tires
71' 4-bolt main block bored .03 over
flat top pistons
268H Comp Cam installed dot to dot
1.6 roller rockers
Edelbrock 5073 aluminum 70cc head set
multi layer steel head gasket
1991 Corvette TPI Intake with enlarge runners, worked base and plenum
Southbay 24lb/hr injectors
255 L/hr in tank fuel pump
1989 Camaro serpentine belt system
Hydroboost brakes
DUI large cap TPI distributor
Electric fans
Larrys Electric conversion harness
No EGR, no CCP, no AIR, no cat
Headers with 2.5 in collector
EBL P4 ECM
Any advice would be great. If that is the nature of my build, I can eventually change it. Sorry for the long post, but I don't know how else to best present my engine problem.
Surging Idle:
The truck when in gear or not in gear will surge up and down 200 RPMs when the car is warming up then for the most part will settle when the truck hits full operating temperature. Occasionally, the O2 sensor will show a sudden drop to 4mV that lasts a second or so. This seems to correlate to the moment the RPM's drop causing the ECM to counter the drop and spikes the RPMs.
I have so far tried to mess with ECMs VE tables to see if there are any unevenness in the Idle areas and it doesn't seem to change where and when the sudden drop of voltage happens in the O2. I also raised the idle rpms from 650 to 700 and found a very slight improvement. I changed the ECMs IAC- stall saver rpms, SA idle high and low parameters and found it to improve the severity of the surging idle.
Hard Cold Starts:
To be honest I am not entirely sure what is going on here, the truck will crank and catch fast when its cold and will run for about a second then just shut off, it will do that 3 times if i don't touch the gas pedal before it can run by its self. Or will stay running the first time if I put just a little pedal in; it will run for a second or two with that slight pedal and then Idle by its self from then on. Another weird thing is that if I start it and get it past the hard couple seconds it can restart just fine then on, even while still being cold. Hot or mildly warm restarts are perfectly fine as far as I can tell.
I have tried to mess with the spark advance added when cranking and it seemed to make it idle better or be less likely to shutoff on me. I have messed with the IAC positions parameters and it just made it worse by increasing the time I have to press the pedal in before it could idle by its self. I have also tried to restrict the PCV flow as that was drawing 5 in of vacuum in the case. It is now pulling 1 in of vacuum. I don't see smoke at startup or during idle.
Poor Highway Performance:
To best frame this problem I am going to go through the 3 stages of builds in this truck. When I first bought the truck it had the same engine I have now and had been rebuilt by the previous owner with an engine swap a 71' 4- bolt main SBC bored .03 over with flat tops. I am not sure of the exact piston to deck clearance but it was really close to the top of the bore. It had the stock 75cc cast iron heads, and a 268H cam and an edelbrock intake and carb. It ran fine but seemed to lack in drivibility and was leaking from a couple of gaskets, so I tore it down for the second build. I put new gaskets and seals on the entire engine and fixed a cracked exhaust valve, dropped a 700r4 in and converted the intake to a TBI unit. Ran great for a while but junkyard electronics for the TBI kept failing and it was lacking in performance on the freeway, so I decided to do a ground up restoration on the truck and do a lot more swapping of parts (I will list below). The truck runs about the same as it did with the TBI unit, but now gets about 3 mpg less than before and like before still looses a serious amount of power if I have to climb a small incline at freeway speeds 70 mph at 1700 rpms and won't respond till I force it to down shift and/or place it a WOT. Around town in runs strong and seems to have a lot of power and gets better mpgs than on the freeway.
I have tried to remedy this by closing down the PCV as it was sucking too much and tried to adjust the timing tables. So this is where I am stumped the most. I can increase the timing up to 56 degrees advance almost everywhere on the table and the truck won't ping, detonate, or change the characteristics of, well anything. I have adjusted the base timing on the distributor so I can advance the timing higher before it jumps terminals. The only time the engine cares about timing is at WOT. I can't advance it above 34 degrees or it detonates a lot. If I drop the timing too far the engine will get more sluggish in any of the areas on the spark table.
Some random facts about the truck that might be relevant. I found the PCV was sucking in oil and was depositing carbon on the valve side of the spark plug, but was burning fine on the piston side; fixed that the other day and doesn't seem to change anything. I am at around 5,000 ft. It cruises down the highway at 70 kPa despite what timing it is at. Idles at about 40-45 kPa. 18 mpg around town and 15 mpg on the freeway. VE learns go in circles depending on the day. Cold days typically removes fuel, warm days it adds fuel.
Drive Train Build Specs:
700r4 with stock converter and corvette servo.
3.73 gears with 34 in tires
71' 4-bolt main block bored .03 over
flat top pistons
268H Comp Cam installed dot to dot
1.6 roller rockers
Edelbrock 5073 aluminum 70cc head set
multi layer steel head gasket
1991 Corvette TPI Intake with enlarge runners, worked base and plenum
Southbay 24lb/hr injectors
255 L/hr in tank fuel pump
1989 Camaro serpentine belt system
Hydroboost brakes
DUI large cap TPI distributor
Electric fans
Larrys Electric conversion harness
No EGR, no CCP, no AIR, no cat
Headers with 2.5 in collector
EBL P4 ECM
Any advice would be great. If that is the nature of my build, I can eventually change it. Sorry for the long post, but I don't know how else to best present my engine problem.
Last edited by Alma Bradshaw; Oct 17, 2019 at 10:20 PM.
Thread Starter
7th Gear
Joined: Oct 2019
Posts: 7
Likes: 0
An update:
I have had to rework the PCV system because I have found that the PCV valve was still sucking a lot of oil in; the entire valve was dripping in oil. I assume this isn't helping the problems I have listed in my first post. So I installed a makeshift baffle that was as large as I could under the valve cover and I cleaned the spark plugs again. That solved the hard cold starts and mostly fixed the unstable idle; It rarely will spike the rpms at idle now.
I still have have the highway lack of power problem. I think the cam might be the wrong combination for this engine. I have noticed that the ebl p4 data loging software shows that while im cruising at 74 kPa at around 1800 Rpms and I start pressing the pedal the pressure hits atmosphere at 87 kPa. Everything runs fine up to this point. After this point and i keep depressing the pedal the O2 sensor starts going lean and the truck will not respond to anymore pedal and the O2 continues to go lean until the transmission downshifts. Then the MAP readings drop to around 70 kPa and O2 readings return to normal and the truck accelerates at a significant rate.
Im thinking the overlap and low vacuum nature that technically makes the 268h comp cam not computer friendly is the problem.
Id love to hear your thoughts and any suggestions to try anything to better understand the problem.
I have had to rework the PCV system because I have found that the PCV valve was still sucking a lot of oil in; the entire valve was dripping in oil. I assume this isn't helping the problems I have listed in my first post. So I installed a makeshift baffle that was as large as I could under the valve cover and I cleaned the spark plugs again. That solved the hard cold starts and mostly fixed the unstable idle; It rarely will spike the rpms at idle now.
I still have have the highway lack of power problem. I think the cam might be the wrong combination for this engine. I have noticed that the ebl p4 data loging software shows that while im cruising at 74 kPa at around 1800 Rpms and I start pressing the pedal the pressure hits atmosphere at 87 kPa. Everything runs fine up to this point. After this point and i keep depressing the pedal the O2 sensor starts going lean and the truck will not respond to anymore pedal and the O2 continues to go lean until the transmission downshifts. Then the MAP readings drop to around 70 kPa and O2 readings return to normal and the truck accelerates at a significant rate.
Im thinking the overlap and low vacuum nature that technically makes the 268h comp cam not computer friendly is the problem.
Id love to hear your thoughts and any suggestions to try anything to better understand the problem.
Last edited by Alma Bradshaw; Nov 17, 2019 at 02:24 PM.
Tech Contributor
Joined: Oct 2008
Posts: 3,729
Likes: 190
From: Custer, WI
What map sensor are you using? Can you verify the "load" when the engine is off to see if the reading is strange?
Cam is not that radical imo and shouldn't have an issue with vacuum
Cam is not that radical imo and shouldn't have an issue with vacuum
Last edited by Pwnage1337; Nov 17, 2019 at 04:33 PM.
Thread Starter
7th Gear
Joined: Oct 2019
Posts: 7
Likes: 0
I am using an AC Delco 1-bar map sensor for a 1991 corvette. The barometric reading at key on and engine off is 87 kPa. That is within +- 1 kPa of the true barometric reading at this elevation of just over 5,000 ft. Everything that I read was that this cam is not too radical either.
Thread Starter
7th Gear
Joined: Oct 2019
Posts: 7
Likes: 0
I am running a knock sensor and i am not receiving hardly any knock at WOT. By that i mean if i do a 2-3 minute WOT pull up a nearby canyon I might get 1 to 8 counts of knock that only pull an average of 1 degree. I find it weird considering I am running 85 octane and it rarely knocks. I don’t feel anything from the engine that would suggest that the sensor is just not picking it up.
Last edited by Alma Bradshaw; Nov 18, 2019 at 06:42 PM.
