C4 Hesitation

[bytor]

Question for the TPI guys. On my 87 Corvette, I have noticed an occasional ‘very short’ hesitation or stumble when aggressively hitting the throttle from a stand still. It doesn't do it all the time. As a matter of fact, It only done it 1 or 2 times in the last month. The car idles perfect, instantly starts hot or cold and runs great. I plan on digging through the service manual and look for the symptom and causes and begin checking things out. Didn’t know if some of you long time vette owners experienced this before and had some pointers.

Here are some of the knowns

- Throttle body clean as a pin.
- New IAC, old one was clicking.
- Checked for vacuum leaks but plan on rechecking.
- Fuel pressure normal
- Checked for injector leak down a few times, system hold pressure.
- Gone through the base idle setup a few times. It’s set perfectly at .54v
- MAF was throwing a code 34. Replaced bad burn off relay and resolved code problem.
- Cleaned MAF and noticed an improvement in throttle response.
- Base timing at 6*
- New plugs and wires

I have not determined yet if it only happens in open or closed loop mode or both.

I have not spent any time looking at the EGR system, look’s like it could be a potential cause of the symptom.

Thoughts or suggestions?

 

[87vette81big]

Make Sure the Ignition wires are spaced 1/4" apart all from each other Bytor.
Factory or aftemarket plug wire loom holders.
Non metallic style.
The C4 Corvette TPI & ECM real sensative to Ignition EMI Radiation.
Found that out on my 87 vert in 2007.
# 5 & #7 Should be spaced at least 1/2 inch apart those ignition plug wires.

 

[grumpyvette]

theres several obvious and a couple less obvious things to check,start by pulling trouble codes, checking for loose electrical connections and vacuum lines.
A steady rhythmic surging,or pulse in the engine idle normally suggests a vacuum leak, or air fuel mixture imbalance issue, a random surging or cutting out at idle generally indicates a electrical or mechanical issue, especially if it tends to kick in as you apply throttle, Keep in mind the word "GENERALLY" as this can and will be only good as basic a guide line to think about. I would check all your vacuum lines,move them around and replace any that are suspect, do the propane torch test to locate vacuum leaks and look carefully at the air filter to throttle body clamps and tubes for splits and clamps for loose or improper alinement , obviously youll want to check see if there are any stored trouble codes, and loose electrical connectors and test sensors resistance or voltage values.
lots of guys , make the big mistake of overlooking partly clogged catalytic converters or defective oxygen sensors because getting under the cars not always easy.
Id start with checking the injector resistance and verify pulse with NOID LIGHTS , check the fuel rail pressure and ignition timing and advance, , ALL SHOULD BE OBVIOUS PLACES THAT COULD CAUSE PROBLEMS , loose electrical connections and grounds are always suspect, the most common cause I find for the lag,well (after verifying its not injector or fuel pressure or OXYGEN SENSOR RELATED or ignition , or knock sensor related,problem )obviously those should be checked , is problems related too the manifold and engine and air heat sensors, those sensors seem to be able to function well enough not to throw trouble codes but not always well enough to allow the engine to run perfectly

THESE THREADS AND SUB LINKS MAY HELP
grab a multi meter and check the sensor resistance and for a good electrical connection

http://www.harborfreight.com/5-in-1-dig ... 98674.html

having a fluke multi meter and an assortment of test leads helps

FLUKE is the brand of choice but they are expensive, shop around, Ive still got the original fluke multi meter I bought in HIGH SCHOOL and it STILL WORKS Ive purchase several imported multi meters from harbor freight , in the last 10 years alone and only one still works
http://www.fluke-direct.com/shop/itemDe ... urer=FLUKE

http://www.fluke-direct.com/shop/catego ... TERS&path=
viewtopic.php?f=80&t=728&p=43477&hilit=camaro+sensors#p43477

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viewtopic.php?f=32&t=2697

viewtopic.php?f=32&t=596

viewtopic.php?f=32&t=10349

viewtopic.php?f=32&t=2697&p=29270&hilit=vacuum+lines#p29270

Re: C4 sensor and relay/switch locations and info


http://www.corvettebuyers.com/c4vettes/ ... mation.htm
EMISSION COMPONENT LOCATIONS

Measured Value
Engine Coolant Temperature Sensor. 185 Ohms @ 210F, 3400 Ohms @ 68F, 7,500 Ohms @ 39 F.
Engine Oil Temperature Sensor. 185 Ohms @ 210 F, 3400 Ohms @ 68 F, 7,500 Ohms @39 F.
Oil Pressure Sender/Switch. 1 Ohms @ 0 PSI, 43 Ohms @ 30 PSI, 86 Ohms @ 60 PSI.
Fuel Quantity Sender. 0 Ohms @ Empty, 45 Ohms @ 1/2 Full, 90 Ohms @ Full.
MAT (Manifold Absolute Temperature Sensor). 185 Ohms @ 210 F, 3400 Ohms @ 70 F, 15,000 Ohms @ 40 F.
Outside Temperature Sensor. 4400 Ohms @ 60 F, 2200 Ohms @ 85 F.
In Car Temp Temperature Sensor. 4400 Ohms @ 60 F, 2200 Ohms @ 85 F.
MAF (Mass Air Flow) Sensor. .4 Volts @ idle, 5 Volts @ Full Throttle.
Oxygen (O2) Sensor. .1 Volt Lean Mixture, .9 Volt Rich Mixture.
TPS (Throttle Position Sensor). .54 Volts Idle, ~ 5 Volts Full Throttle.

Sensor Locations

Sensor


Location
Engine Coolant Temperature Sensor. Front of engine, below Throttle Body.
Engine Oil Temperature Sensor. Left rear of engine, just above the oil filter.
Oil Pressure Sender/Switch. Top, left hand rear of engine.
Fuel Quantity Sender. Top of fuel tank, beneath filler pipe escutcheon panel.
MAT (Manifold Absolute Temperature Sensor). Underside of manifold air plenum at rear.
Outside Temperature Sensor. Right side of engine, top right corner of radiator.
In Car Temp Temperature Sensor. Coupe: above left seat near interior courtesy light, Convertible: center of cargo compartment lid.
MAF (Mass Air Flow) Sensor. Front of engine ahead of throttle body.
Oxygen (O2) Sensor. Left side of engine, in exhaust pipe.
TPS (Throttle Position Sensor). Right side of throttle body at the front.

 

[bytor]

Make Sure the Ignition wires are spaced 1/4" apart all from each other Bytor.

Thanks for the tip. I did some rerouting on the plug wires to clean things us some. I had a few that were touching.
I also played around with TunerPro and made a few log captures while starting from a stand still at low to mid throttle. Of course the car performed perfectly with no hesitations. Below are the graphs. I don't see anything out of the ordinary temps look good, TPS signal clean, fuel enrichment happening when I step on it and MAF tracking RPM. In the second graph however, there is a little blip in the MAF gps data towards the end of the run. But, if you look at the MAF raw data tracking just behind it, it's good. I'm assuming it's a logging or communications error but I will continue to play around with it.

 

[87vette81big]

Check & reclean the MAF SENSOR Terminals Bytor.
They must be Spotless.
Clean gently with a piece of 400-600 grit Emery Cloth or Sandpaper.
Apply a light coat of Silicone Dielectric Grease.
Retest the same.

 

[bytor]

Thanks for the feedback, I will check that out...

How do you clean the female side of the connector?

 

[87vette81big]

There is no easy way to clean the Female terminals Bytor.
You could try an Electronic TV Tuner Aerosol cleaner.
I have a terminal repair kit I use sometimes. Remove one terminal at a time & try to wire brush clean.
If its nasty corrored replace entire Connector with new from NAPA.

 

[bytor]

Cleaned the MAF connector, it wasn't in bad shape with only a little tarnish. Nice and shiny now though..

While looking at one of my last TunerPro logs. I noticed at the very end while I had the car at idle in the garage, the IAC counts dropped to 0. That didn't seem right so I made a few more logs to confirm and sure enough it would eventually drop to 0. Below is the log showing this. At the very end you can see the IAC going through it's reset procedure at key off. Kinda cool.



One cause for this would be a vac. leak. I double checked for vac. leaks and found none. I noticed my PVC hoses are a bit hard but they are not leaking. I will replace them anyway. I will double check all my intake runner bolts and make sure they are not loose.

The other cause could be an incorrectly adjusted base idle. I have verified the base idle per the service manual procedure a few times. Just for fun, I backed off the idle set screw 1/4 turn and logged the results. You can see below after the throttle blip the IAC is now holding at 7 and the MAF came down a bit as expected. The idle RPM dropped by 25 rpm as well.



The service manual says the normal idle scan data range for the IAC is 1-40 counts and the MAF is 4-7 grams. I might back the idle screw off another 1/4 turn to see if I can split the difference on these.


.

 

[bytor]

Backed off the idle screw a little bit more and now have my IAC counts at 18. The car seems to be happy with this and the idle seems just a hair smoother. I'm not sure that the IAC at 0 contributed to my random hesitation but I guess it could. I have not had the issue again since I started this thread.

I double checked for vacuum leaks and found none.

I did spend some time checking out my throttle body shaft for wear. It does have some 'slop' in it. I can wiggle it around some but haven't actually measured it. I don't think it's a significant vacuum leak because I did the propane test with no indication of RPM/MAF increase. One thing I have noticed since I have owned the car is a stiff throttle just off idle. It's not sticking, just doesn't operate a smooth as I think it should. It's only an issues when the engine is hot. To me, it's an indication of the throttle shaft wear.

I did some quick looking around and noticed Holley and BBK make new 52mm. I had rather stick with the stock 48mm size. Do you guys know of any place that rebuilds and install bushings/bearings in the stock units?

 

[grumpyvette]

viewtopic.php?f=55&t=290&p=22467#p22467

https://www.youtube.com/watch?v=S_ni39iwAys

Ive had new bushings and throttle blade shafts installed in older TPI throttle bodies ,the cost locally was at that time parts & labor near $170 Id never do it again, the BBK 52mm-or 58MM throttle body seems to be at least to me a better deal in every respect.
http://www.summitracing.com/parts/sum-227702
summit sells a 58mm throttle body, obviously youll need to verify it fits and works in your application
now I have had my bbk 58mm throttle body on my 383 , used both on an extensively ported TPI and Holley stealth-ram intake, both as delivered and later with several custom plenum designs , and after using it id be hesitant to go back to a factory throttle body , it just seems to operate better and while it may not be a significant increase in power it looks better, it operates smoother and I'm sure it flows more air. keep in mind the throttle body's basically an AIR GATE and while you may not need its full potential, its not in control of your engines fuel/air ratio control, to near the extent a larger carburetor on a stock carb intake would be, simply because the computer controls the injector pulse duration and sensors control the fuel/air ratio, and the TPI plenum and runners are restrictive, so theres just no way adding a larger throttle body hurts mid and upper rpm power, yes it might slow off idle responsiveness very slightly but if your building a kick-butt, performance engine just the upgraded cam duration already does that to a larger extent.also notice the 58mm throttle body has the air foil machined into its basic design

viewtopic.php?f=32&t=10349&p=42530&hilit=relocating+sensor#p42530

 

[87vette81big]

The stock L98 48mm throttle body has been debated for 2 decades now Bytor.
48MM X 2 unit many claim can support up to 500-550 Flywheel HP.
Some even say 600 HP.
I don't know what VE Volumetric efficiency they are working with.
If your 1987 C4 coupe is stock I think its best to stick with the stock 48mm unit.
See if you can find a low mileage part local or Fleabay.
$20-50 bucks.
There have been issues with some owners of BBK Units.
On the Vette Forums.

I am happy with the stock 48 mm X2 on my 87 Vert.
I gave the linkage a shot of light lubricating spray oil.
And Throttle shaft.
Smooth as silk operation.

 

[bytor]

The stock L98 48mm throttle body has been debated for 2 decades now Bytor.

I was wondering what the additional size of the 52mm over stock would do to the off idle and low end throttle response. It would seem that you would lose some resolution or responsiveness at low throttle openings because your betting more air at a given throttle position than you were before with the 48mm stock unit. Then again, the difference may be unnoticeable.

Here's a rant I ran across on another forum. Interesting prospective. I would not be doing this for an HP gain. The idea of sealed throttle shaft bearings is the only driver for considering an aftermarket TB, just wish they made a 48mm one. I also agree with Grumpy about having a stock unit rebuilt, probably doesn't make since in the long run.

Re: 52mm throttle body on a stock TPI?
I personally do not care for larger throttle bodies on street cars. In most applications (especially mildy modded street cars) they're a complete waste of money. I can tell you for a fact, that installing a larger throttle body on your car will give you NO power increase what so ever. Zip. Zero. Nada. Nothing.

What it will do, is destroy your throttle response and may even give you a "stumble" or "bog" off the line. That is what happened with my LT1 Formula ( http://www.cardomain.com/id/lt1fun )
after installing an Edelbrock/BBK 52mm throttle body and it took me forever to finally figure out what the problem was. It was such a frustrating time trying to figure out why my car developed this stumble, out of the blue.

I kept getting this "bog" off the line, where there was a slight pause between the moment I hit the gas, and the point where the car actually stated to accelerate. The car would always stutter whenever I'd give it 1/4 throttle. At the time, I figured it was ignition related so over the following couple weeks, I eventually ended up replacing ever part of my ignition system which (back then) cost my well over $1000 CAD.

Still didn't solve the problem. It wasn't until I met up with an old friend of mine (who happens to be a mechanic) that I finally figured out the problem. I explained to him the symptoms and he asked me what was the last thing I did to the motor before I started having problems with throttle response. After thinking about it, I finally remembered that the throttle body was the last thing I did to the car and he said "BINGO! That's your problem right there."

He then explained to me about how/why larger throttle bodies often cause drivabilty issues and suggested I remove the 52mm and re-install the stock 48mm. I was hesitant to do so at first, mainly because I had just finished spending $450+ on this new fancy throttle body and it was such a really nice looking piece. But I was desperate to find out what the problem was and willing to try anything. I figured if the 52mm throttle body wasn't the problem, I could always just put it back on.

To make a long story short, immediately after installing the stock T.B. back on the car, all drivability issues disappeared. Throttle response was smooth and crisp. No bog. No stumble. The car was right back to the way it used to be.
I was happy but at the same time pretty pissed. I mean, on the one hand, I finally solved the drivability problems I was experiencing. On the other hand, I came to the sad realization that I had just spend $450 on a part that didn't give me one single horse power and instead ended up costing me over $1000 in ignition "repairs" that were never even needed....

For the record, I did put the car on the dyno after installing the larger throttle body (I always put my car on the dyno after upgrading anything)
and after 4 runs, the car didn't make any more HP than before. This was on a heads/cam LT1 mind you (with a ported intake manifold), and I was putting more than 360+ at the wheels N/A and over 500RWHP on the spray. On both passes on (and off) the nitrous, I gained absolutely NOTHING !!!

Needless to say, i was pretty disappointed with the results and I tried to comfort myself in the fact that even if I didn't gain any HP, at least the new throttle body looked cool and gave the car a "sucking sound" through the intake that made it sound like I had a blower on the car

Looking back now, it was a learning experience for me.

There's was a lot of dumb things I bought for that car that were a complete waste of money. I guess I was just suckered into the magazine ad's and internet talk and just believed a lot of what I read. Now with my IROC, I'm going off of personal experience and only buying parts that I know are actually worth it.

A larger throttle body definitely is NOT one of them.

A stock 48mm TPI TB flows aprox. 780cfm, which is already more than what a stock (or aftermarket) TPI set-up could ever hope to take advantage of. Really, there is virtually no reason to upgrade to a larger throttle body if you're running a TPI intake.

It's your call really but I wouldn't recommend a larger throttle body at all.
I've never seen anybody gain any HP out of a larger throttle body other than those with highly modified motors with power-adders such as a Turbo or a supercharger. A TPI motor will not benefit from a larger TB, I can assure you.

For the money you'd spend on a 52/58mm T.B. you could upgrade to a set of 1.6 roller rockers and pick up a guaranteed 15-20HP.
You could get a custom Chip burned for your car. A set of headers, cat-back etc. etc. etc.

 

[87vette81big]

Its the inherit. nature of GM TPI Bytor.
Peak VE occurs at a low stump pulling range.
Dictated by ultra long runner lengths.
To me its not worth spending thousands on to upgrade.
Dedicated other engine build better. No TPI.

 

[bytor]

No more hesitation since I adjusted my IAC correctly so it wasn't at 0 during idle. I ended finding a local carb shop that reamed and installed shaft bushings for $40. Very pleased, all the slop is gone and smooth as silk now. The gasket kit I used had a nice throttle body rebuild doc so thought I would share.

Plus some good YouTube how to's
part1 http://www.youtube.com/watch?v=juXZpZ6zSbA
part2 http://www.youtube.com/watch?v=-Zw-Wba-E8U
part3 http://www.youtube.com/watch?v=igxDCZR8goc&list=UUwq9nV2zUDs7NjSiY39U7_g
part4 http://www.youtube.com/watch?v=z3kSkiQckYM