I've been battling a really bad driveability problem since August last year and now purchased a temp gun to figure it out.

At idle the temps vary wildly between cylinders, is this normal? I get:

---| |---
410 | | 455

520 | | 455
420 | | 680

540 | O | 620

Is there any way I can use the gun to isolate the misfiring cylinder?

I've pulled the spark plug from the hottest cylinder, pictures attached. Car had only ben slowly driven and was idling before that, but can you still tell something?

EDIT: Just added a photo of it next to one from a much cooler cylinder. Also just noticed one is an AC R44T, the other a NGK BR5FS

EDIT again: Just pulled #8 and it looks oil soaked, see attachement. Could this be the cause of the misfire?

66 views and no-one knows...

What engine is this and are you using any oil. What gap are you using and what ignition. Also what is your timing set at. Just looking at those plugs I'd say they are due for a change anyhow.

it would be better with more info about your engine and mileage, etc. Those oil deposits would definitely cause a misfire. Looks like your valve seals are getting old and possibly causing the problem. Not a huge deal, get some new plugs and change the seals. if you really want to be cheap just change the two on # 8....

It's a stock, 8.3 compression '73 SBC. No idea about mileage. Stock spark plug gap with new HEI. Timing is 17/32 plus 21 vac adv. Those plug have done 2-3000 miles.

It uses oil, yes. This problem started suddenly last August although it was using oil before that.

Don't particularly need cheap solution, just don't want to try and fix the wrong thing :) Do I need to take the springs off to change the seals? If so I'll change the springs as well as they're probably 40 years old...

So people agree it's oil in the chamber causing the misfire?

obviously, do the easy things first. complete tuneup (plugs, cap rotor, timing), change the fuel filter, be sure you have all good plug wires (if in doubt, change them too). set timing correctly, then adjust the carb. misfire under load would lead me to check out fuel filter/pressure (do you have a fuel pressure gage?) then plug wires. most likely causes right there (assuming the tune up is correct).

mine just did the same thing the other day. did a tuneup and started it back up. seemed ok till i noticed the fuel pressure was reading 3 psi at idle (normally about 8psi). opened my fuel filter up (it has a replaceable element) and it was completely clogged with trash from the tank. just got it painted and i think they may have pulled the tank filler and left it open during the bodywork getting crap in there.

Thanks, the carb is fairly new and my A/F gauge never responds to the misfire and sits pretty steady under all driving conditions. And my fuel filter is new, see-through and completely clear.

The HT leads are fairly new MSD 8.5mm and have only done about 8000 miles. I did however, buy a new set but the came with the wrong type of boot so replacing them at the moment which is incredibly difficult. I've changed #3, #5 and #7 so far.

EDIT: Coil and rotor are brand new, changed them two weeks ago to solve the misfire

it may be a longshot, but the mix matched plugs could be a part of the problem.

Don't forget it's over-heating as well btw. Could any of these problems cause that? I'll order some new NGK spark plug as well.

Don't forget it's over-heating as well btw. Could any of these problems cause that? I'll order some new NGK spark plug as well.

you didn't mention that. overheating the coolant? temp gage going up? how hot? or do you mean some cyls are running hotter than others?

any oil in your coolant?

Sorry, thought I'd mentioned that in my initial post. The over-heating has been getting worse and worse since August and is speed/rpm dependent. It used to be constant between 180-190 but if I go anywhere over 50mph now, I can't keep it under 210. It doesn't over-heat when idling.

It's a mechanical/hydraulic gauge fitted in the manifold by the thermostat.

P.S. Coolant is clean.

where is your timing set?

where is your timing set?

Timing is 17/32 plus 21 vac adv. I've tried advancing it a bit but it has no effect on the temperature

have you done a compression or leakdown test?

have you done a compression or leakdown test?

Did a compression test when I put the last set of plugs in and it was fine.

hmmm sounds like you checked all the things i would think of... do you have a spare coil laying around? reason i ask is i had a coil go bad and it acted like you are saying. thing was though, you could see the tach jumping all over the place at the time. might be worth swapping out the coil if you have a spare or one you could borrow. otherwise, i can't really think of much else. oil in the cyl shouldn't cause a bad misfire unless there is a lot of oil and the plug is getting fouled by it.

you verified no vacuum leaks? what about fuel pressure?

I assume vacuum leaks would be negligible under load? It idles fine, 21Hg vacuum and A/F gauge reads fine.

As mentioned I replaced the coil a couple of weeks ago and it didn't help. Tach is steady.

Could a bad valve spring, lifter or flat cam do this? I ran dry of oil a couple of times last year and just wondered if that could have caused it?

Anyway, will proceed with replacing HT leads and plugs. I'll also take the valve cover off but what do I look for?

yeah a flat cam or any valvetrain issues will do that. I was kind of avoiding that bad news though. easy enough to check... pull the valve covers and turn it over by hand. if you don't notice any rockers that are not moving or are moving less than the others, you will most likely need a dial indicator to tell. if you are eating a cam lobe, your oil will most likely have some small sparkly metal bits in it.

you said it misfires under load. does it depend on the RPM, or does it do it anytime you get into the throttle regardless of RPM?

It gets much worse with RPM. Can sometimes handle load at low speed. Odd thing is on some days it's absolutely fine and and will go full throttle through the entire RPM range. That's rare though.

Would valve issues cause over-heating as well? That's kinda why I dismissed that theory

FYI,R44T is not the stock plug for most 73 350sbc apps unless maybe its a low geared truck with high cruise rpm and high load carry/pulling situations..

The correct plugs for most car 350sbc apps in 1973 were a 1step hotter extended tip R45TS AC Delco srark plug.

From the look of your plugs you could go 1 step hotter too so id try a new set of R45TS plugs gapped to .045" elec ign or .035" points ign to see if that stops higher rpm/hgher load misifres.

If notlook for bad plug wire/s poss arcing to headers under load ,loose or cracked dist cap,loose contact on rotor,bad coil because ign issues often cause misfiers under loaed.

But higher rpm /higher load misfires can also be caused by fuel delivery issues ,bad-broken valve springs etc so keep that in mind too.

But in these case always go after the easier/cheaper ign items 1st to ensure thats not the issue which is often the case before moving on to more expensive/more involved diagnostics outside the ign system.

BTW,some of your plugs are very dark so maybe when getting in some traffic for a while gettting them more dirty/fouled then getting on after that it is then most likely to misfire with a few very dirty plugs . But after crusing for a while on open road plug/s can clean up a bit making it less lkely to misfire when getting into it after the motors been at higher cruise rpm for a while.

Try a new set of r45ts plugs before going any further.

Scott

FYI,R44T is not the stock plug for most 73 350sbc apps unless maybe its a low geared truck with high cruise rpm and high load carry/pulling situations..

The correct plugs for most car 350sbc apps in 1973 were a 1step hotter extended tip R45TS AC Delco srark plug.


From the look of your plugs you could go 1 step hotter too so id try a new set of R45TS plugs gapped to .045" elec ign or .035" points ign to see if that stops higher rpm/hgher load misifres.
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Hi, I got R44Ts because that's what's listed in the Haynes Chevelle manual for this engine. However, the 76 engine has R45TS listed and I've never really been sure if this engine is a '73 or '76. Would I be safe with a R45TS either way? Only thing is I picked up my new R44Ts a couple of days ago and can't refund them so want to be absolutely sure they are wrong.


But higher rpm /higher load misfires can also be caused by fuel delivery issues ,bad-broken valve springs etc so keep that in mind too.


Took the valve covers off and couldn't see anything obvious and the oil looks fine.


But in these case always go after the easier/cheaper ign items 1st to ensure thats not the issue which is often the case before moving on to more expensive/more involved diagnostics outside the ign system.


Yeah, I've replaced the coil and rotor, dist is fairly new, replacing HT leads and plugs atm.


BTW,some of your plugs are very dark so maybe when getting in some traffic for a while gettting them more dirty/fouled then getting on after that it is then most likely to misfire with a few very dirty plugs . But after crusing for a while on open road plug/s can clean up a bit making it less lkely to misfire when getting into it after the motors been at higher cruise rpm for a while.


Motorway cruising never helped, just made it worse...


Try a new set of r45ts plugs before going any further.


Are you absolutely sure? Also, can anyone verify that broken valve stuff can cause overheating?

Thanks

you said you pulled the valve covers and didn't see anything appeared to be wrong. turn it over by hand and verify that all your valves are moving the same. you should be able to see if you have any broken springs or valve issues. unless it is really obvious, it might be tough to tell if you have a flat cam lobe.

again, do you have a fuel pressure guage on the car? if you have tank/pump/filter/dirty needle/seat issues, it will misfire under load too. mine did that last week in fact. easy fix for mine since my fuel filter is able to be dissassembled. cleaned it and it was good to go.

But obviously, like Scott said, check the easy obvious stuff first.

on the plugs, even if you have the wrong heat range in there, i doubt it would misfire right off the bat with new plugs. it would take a bit still they started to foul. keep us updated.

I'd done 5-6000 miles on those plugs before the misfire started. And I was going to turn it over checking valve movement but then thought without oil pressure it probably wouldn't tell me much?

I'm fitting new plugs and HT leads as we speak. The carb is quite new, I changed jets several times last year and it all looks clean. I'll check the float level.

If it was fuel supply I'm not sure it would misfire the instant I got on the throttle.

This might be really off the mark, but you mention installing a new HEI distributor. Was it a points distributor originally. If so, was the hot lead from the firewall to the distributor also replaced? If not, there is still a resistor wire in there and your not getting enough voltage to fire properly. My other idea is that the distributor may be installed improperly. See the attached: http://www.chevelles.com/techref/ftecref5.html , hope this helps. Keep us posted and good luck.

Thanks, there's no resistor in the supply, have re-wired it myself.

I might have some good news-ish, as I fitted the remaining five HT leads and replaced all plugs today and took it for a spin. No misfire! :) I still need to take it for a proper drive though and it still got slightly too hot as well.

I'm not 100% happy though, as I don't think I should have had this problem in the first place, with HT leads and plugs that's done less 5-8000 miles. Hmm... And the oil on the back plugs don't give me confidence either.

I've been considering a set of new heads and a new cam since I got the car four years ago, so might go for it soon anyway.

Anyway, thanks a lot for all the help and advice here :hurray: :thumbsup:

Glsad t see you got it corrected,like suggested misfire under load is often ign related like dirty plugs or bad-poor quality wires/coil/cap/rotor issues too.

Did you use the 44 or 45 heat range plugs?

I ask that because if you went with 44's a agin they will likely get sooted up the fuutre again just lke tis last set did because you havent done anything to correct/change the fuel calibration or freshen up the motor oil wise (rings-guides-v seals) to clean up the plugs up some or if you didnt go up the 1 step hotter 45 plugs to help burn off some of additonal fuel & or oil & 1 step hotter plugs would not be an issue in your motors case.

A spark plugs heat range also has to do with the motor's fuel calibration/carb setup and ability to keep oil utof the comb chambers etc and it's not just mfg's rec plug for application being run that se4t in stone for a plugs heat range.

In your case with 44's looking kinda sooty and or dark -med bowrn at best you could easily go 1 step hotter from 44's to 45's without worry of being too hot IMHO .

So keep this in mind if you have this issue again in the future if still running the 44's.

Scott

Hi,

I used NGK BR5FS. Just looked up the conversion chart and it's the same NGK plug for both AC R44T and R45T. Why?

To lower amont of plugs having to be mfg'd & stocked they now use 1 plug to cover 2 heat ranges.

It's std op proceedure these days and in general isnt that much of an issue because there isnt a big dif in going up or down 1 step in heat range in most cases.

Scott