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C4 Spark Plug Confusion

89C4KB

Member
Joined
May 30, 2019
Messages
12
Location
Cameron Park, CA
Corvette
1989 Bright Red Coupe
I recently purchased a 1989 Coupe with 42,000 miles. It did not come with any service history. It currently has a minor vibration from the engine at low to mid rpms. It goes away and runs hard when I get into the throttle. I pulled the air filter and found it was black. Judging by this I feared what else I might find. As much as I was scared, I pulled the diptsick and found the oil looked like it was put in the week before. I am fairly certain the the plugs, plug wires, rotor and distributor cap are all factory (based on the mileage).

This is why I am starting at the air filter and working my way back to the distributor with new parts. That said, I picked up a set of ACDelco Rapidfire 5 plugs (ACdelco part number 19308030). Guy at the parts store told me those were the correct plugs for the vehicle and they were pre-gapped. I got home and found all the plugs gapped at .043. Then I pulled out the vehicle manual and found the specs call for .035 gapping.

I proceeded to gap all of the plugs to .035 then went to bed. The plugs are still on the work bench. When I woke up I had a bad feeling I was sold the wrong plugs due to the consistent gapping they all had. I started searching forums, here and elsewhere, and found these plugs are correct for my vehicle. Further reading showed people running these plugs gapped anywhere from .035 to .050 with good results.

So bottom line is...I am going to install these plugs tomorrow and don't know what I should gap them at. I am thinking of gapping them at .040 to split the difference of everyone else's experience with these plugs, and see what happens unless I receive information to the contrary.

I am a first time Corvette owner so please start yelling at me if you see something horrible with this plan.

Thanks
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
I would gap them at .045, and use synthetic engine oil or anti seize on the threads. I would also do the cap and rotor asap if they are OEM.
 

89C4KB

Member
Joined
May 30, 2019
Messages
12
Location
Cameron Park, CA
Corvette
1989 Bright Red Coupe
I probably should have been more clear when I said new parts front to back.

So far I have replaced the air filter and cleaned out the MAF sensor.

Tomorrow entails new plugs, plug wires, rotor and distributor cap.

Followed shortly thereafter by oil and oil filter. Even though it's clean, I don't know what kind of oil it is.

After looking around the engine bay, I can't say I am looking forward to changing the plug wires. But I figured at 30 years old they are likely dried out and in need of replacement. Anyone have any tricks for this task?
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Do one plug wire at a time and don't be in a hurry. I would hold off on the plug wires, unless they are damaged, until after installing the other parts and test driving.
 

89C4KB

Member
Joined
May 30, 2019
Messages
12
Location
Cameron Park, CA
Corvette
1989 Bright Red Coupe
Update, but i still need help!

I got all the plugs changed this morning. That was not the easiest thing I have ever done, but not impossible either. From what I had read I was fearing the number 8 plug, however that one was not so bad. The number 2 plug gave me the most trouble, until I found someone on here stating to remove the inner wheel well skirt. Why didn't I think of that;shrug. That's why I am loving this site.

All of the electrodes looked OK, for 42,000 miles, with a light sandy color to them. I was happy to see that none were fouled. The top half of the number 8 plug snapped off as soon as I started turning the plug socket, so I am assuming it was cracked before any force was exerted on it. The number 4 plug also snapped, but after evaluating my performance, I think that was probably me and the wobble socket I was using.

I fired it up and took it for a test drive. The engine ran a lot smoother and seemed to have a little more giddy up. I was enjoying my morning drive with my cup of coffee and newly smoothed out engine when I looked down at the coolant temperature gauge and saw LO. The car had been running for about ten minutes at that point so I was sure the temperature was not low. I checked the oil temperature which was running at about 190. From what I have seen so far, both of these temps run in the same neighborhood. I promptly turned around and headed home to let the car cool off so I could start looking around. I verified my suspicion the car was not running cool by grabbing the radiator hose. Definitely not running cool.

I am guessing I likely knocked a sensor connection loose in the engine compartment while changing the plugs. Can anyone point out some areas for me to check?

Thanks
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Check your coolant surge tank and see if it is low on fluid, or the sensor is unplugged or damaged.
 

89C4KB

Member
Joined
May 30, 2019
Messages
12
Location
Cameron Park, CA
Corvette
1989 Bright Red Coupe
Surge tank is between the hot and cold marks. The sensor was not unplugged at the manifold. I traced the wires back but lost them into a loom. I was at the auto parts store picking up the distributor cap and rotor so I grabbed a new sensor. I installed that and it still reads LO.

I noticed right before this started that the car took more cranking before starting, don't know if that has anything to do with it.

LLC5, thank you for the time you are throwing at this, I really appreciate it.
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Surge tank is between the hot and cold marks. The sensor was not unplugged at the manifold. I traced the wires back but lost them into a loom. I was at the auto parts store picking up the distributor cap and rotor so I grabbed a new sensor. I installed that and it still reads LO.

I noticed right before this started that the car took more cranking before starting, don't know if that has anything to do with it.

LLC5, thank you for the time you are throwing at this, I really appreciate it.


There should be a sensor in the coolant surge tank to tell you when coolant level is low, does yours have one?
 

89C4KB

Member
Joined
May 30, 2019
Messages
12
Location
Cameron Park, CA
Corvette
1989 Bright Red Coupe
I have looked around the top and sides of the tank. There is no sensor, or anywhere for a sensor to mount. There are no wires going to the tank either. The only thing going to the tank is the hose from the radiator neck.

Edit:
I believe I might be referring to something different than you. I looked around at different parts sites and found a "coolant recovery tank" that fits 84-96 models (which is what I was referring to). I also found a "radiator surge filler tank with sensor" for 90-96 models. I did not see any surge tanks for 84-89 models.

Based on this I do not believe I have a surge tank. Is this correct?
 
Last edited:

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Correct, you may have a reservoir and not a surge tank system. Does your temp gauge read correctly? If not find where your temp gauge sensor is located (usually on a cylinder head, but not always) and check the wiring and sensor for damage.
 

89C4KB

Member
Joined
May 30, 2019
Messages
12
Location
Cameron Park, CA
Corvette
1989 Bright Red Coupe
My temperature gauge read LO (digi dash) and never changes. The temp sensor is located on the front of the intake manifold. The connector was connected and I could find no damage to the exposed wiring before it entered the plastic loom. I installed a new temp sensor and it still reads LO.
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
My temperature gauge read LO (digi dash) and never changes. The temp sensor is located on the front of the intake manifold. The connector was connected and I could find no damage to the exposed wiring before it entered the plastic loom. I installed a new temp sensor and it still reads LO.



Disconnect the sender wire, and with the key on ground the wire with a jumper and see where your gauge reads. Note both disconnected (open circuit) and grounded (circuit to ground) gauge readings with the key on (ignition, not accessories).
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
I'll try that when I get back home with the car on Monday.


Thanks



If this happened right after changing the plugs, you might have the wrong temp sensor location for the gauge.

Double check for any sensors or wiring on the cylinder heads (between/near the spark plugs) for damage.
 

89C4KB

Member
Joined
May 30, 2019
Messages
12
Location
Cameron Park, CA
Corvette
1989 Bright Red Coupe
Problem Solved!

LLC5, thank you for the follow up advice and helping me keep it simple.

Last night I grabbed a flashlight and went looking around the heads. It turns out there was a yellow spade connector on a sensor between the 6 and 8 cylinders that was cocked at a 45 degree angle. I disconnected the connector, then re-connected it and started the car. The dash still read LO, however that is what I expected since the car had been sitting in the garage for two days. Once I heard the RPMs drop, I looked at the dash and saw 185.

Thanks for all your help on this one.
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Glad to hear you got it, good job.
 
Joined
Jan 28, 2020
Messages
13
Location
Mid Hudson Valley NY State
Corvette
1988 Black Coupe Auto Z52 17k Miles
I recently purchased a 1989 Coupe with 42,000 miles. It did not come with any service history. It currently has a minor vibration from the engine at low to mid rpms. It goes away and runs hard when I get into the throttle. I pulled the air filter and found it was black. Judging by this I feared what else I might find. As much as I was scared, I pulled the diptsick and found the oil looked like it was put in the week before. I am fairly certain the the plugs, plug wires, rotor and distributor cap are all factory (based on the mileage).

This is why I am starting at the air filter and working my way back to the distributor with new parts. That said, I picked up a set of ACDelco Rapidfire 5 plugs (ACdelco part number 19308030). Guy at the parts store told me those were the correct plugs for the vehicle and they were pre-gapped. I got home and found all the plugs gapped at .043. Then I pulled out the vehicle manual and found the specs call for .035 gapping.

I proceeded to gap all of the plugs to .035 then went to bed. The plugs are still on the work bench. When I woke up I had a bad feeling I was sold the wrong plugs due to the consistent gapping they all had. I started searching forums, here and elsewhere, and found these plugs are correct for my vehicle. Further reading showed people running these plugs gapped anywhere from .035 to .050 with good results.

So bottom line is...I am going to install these plugs tomorrow and don't know what I should gap them at. I am thinking of gapping them at .040 to split the difference of everyone else's experience with these plugs, and see what happens unless I receive information to the contrary.

I am a first time Corvette owner so please start yelling at me if you see something horrible with this plan.

Thanks

I know this is an "old thread" but being I just swapped out all the plugs on my `88 L98 - I noticed all the new plugs out of the box were gapped at .040 -.045 using my machinists calipers. I checked my handy ring gage gap tool and the .035 mark was off a few thousands undersized so I had to re-gap all he plugs by tapping hem a bit to close the gap a couple of thousands. I know I'm nit picking but I am a retired machinist/machine shop manager so these things bother me. I like perfection (to a a fault sometimes) but I think anywhere between .035 and .040 is probably good enough for horseshoes !
 

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