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1985 intermittent misfire when cruising


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Yep been a while since I have had to come back after the weber install. Truck has been through everything and moving from Cali to TX. And since I lost my '11 Silverado last year (t-boned not my fault) I have had to resort to daily driving my 85 once again. Been running fantastic other than it getting shot at and had to replace the radiator and remove the A/C condenser (good thing it didn't work in the first place😒) And a new alternator after the junkyard find went bad after 4 years. Along with the 3rd set of u-joints.. Now discovered a new issue in the last week that is throwing me for a loop..


Doesn't happen at idle, or at WoT. Only when cruising over 50 ish. i hear a weird "pop" sound from the motor and it will only happen 3-4 times in a drive.(about 5 miles). Some drives it doesn't happen at all. Seems to be less likely to happen in mornings or nights. Its not consistent with when it happens / doesn't have a rhythm. No over-heating or oil/coolant mixture occurring. I did notice that when I pulled the oil cap there seems to be a decent amount of blowby coming out, truck always had a little but it seems a bit worse now. I changed the fuel filter and checked my FPR and gauge and it is getting just over 3psi of pressure right up to the carb and re-adjusted the carb due to it running a tad lean. changed the Exhaust side ignition coil because the original had looked like it was leaking oil? Made sure all my wires were going to correct sparkplug. Inspected vacuum lines where I had installed caps due to the weber swap and checked the dist. advance line and it looks good. I have posted pictures of the spark plugs because I don't know how to read what they are supposed to look like in pictures. They are NGK bpr5 and 6 installed about 2017 along with wires. I am not sure about distributor cap or rotor and have looked into the ignition module but before I spend any more money would love to have some opinions on where to direct the problem to.

 in the order of, #1E #2E #3I #4I





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Hopefully NGK. I don't care for the iridium ones much.



They look like the mixture is ok but judging by the deposits, 1 and 2 are burning a bit of oil. Could just be the intake valve seals.



What do the intake side look like?


Plug gap is 0.032" so don't run them any wider.

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ill pull them tomorrow and see. are valve seals easy to fix? I have not messed with timing much on vehicles only main thing ive done is timing chain on a 305. I'm worried about how deep you have to get into the motor that I mine as well replace the head gasket and chain tensioner while its there since I know the tensioner is bad as it rattles on cold starts when truck is low on oil due to pan gasket leak that I have never gotten into because it looks like you have to take the whole front end off to get it out.


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Valves will rattle when cold if their clearance are set loose. I would check them first.


Unless blown leave the head gasket alone. Z24s tend to blow their head gaskets every 100k The preventative measure is to re-torque the bolts on a Z24 once a year. Loosen off only one bolt and right away tighten to 60 ft lbs. Go to next bolt, any order you like. Only one bolt loose at a time!


You should be running an oil high in ZDDP like Rotella T4 and a good filter like WIX or NAPA Gold. For Texas go 15w40. Rattle on startup is no big deal.



For valve seals loosen all the bolts in stages on the two rocker shafts and lift away. Valves are all now closed. Put about 2 foot of nylon rope into the exhaust spark plug hole. Tie a big knot on the other end that won't allow it into the cylinder. Turn crankshaft bolt with a ratchet till the piston comes up and compressed the rope tightly against both closed valves. DO NOT use the starter for this!!! Valves cannot fall in, so compress the spring, remove keepers and lift away. Pry old seals off and push new seals on. Replace the springs, turn engine backwards to release rope go to next cylinder. When done, replace the rocker shafts tightening bolts in stages. Done.



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the last time i re-torqued headbolts was probably 2-3 years ago so would be a good time to do it again. 

I just throw any 30 weight oil in the truck because the thicker oil doesnt leak out of the pan as bad. (only about a quart a month but still annoying) I do use the napa gold filter thats roughly 8-10k miles old now. Could change that i guess 😛


Is there any specific brand for the valve seals and springs that you recommend? That was another thing I was thinking that I was getting a stuck valve due to a gunked up spring somewhere because im almost sure that this motor has never had any of that replaced. I just hope that if that was the case that none of the valves tapped a cylinder.


I do appreciate the quick response and how-to! 

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Change the oil and the filter every time and every 7k miles under normal conditions. Keep the oil changed, and nothing will stick or get gunked up. Rotella T4 is a diesel oil which is more detergent than gas engine oils so it will clean the engine even more.  Get that oil pan fixed first.

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Went ahead last night and ordered new plugs, cap and rotor. valve stem seals and valve springs. along with pan gasket and a new filter. If all this doesn't fix issue will do a compression test and see if it is a cylinder.. hopefully not fingers crossed. Just trying to rule out all the smaller things before having to make a decision if I had to.

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A couple of misses is alright. Pop sound might not even be a miss.


Don't really need new springs but...


NGK ? BPR6ES and BPR5ES? Trouble with the Iridium is you can't clean them without wrecking them.


A compression test is a good idea. I think you will find it alright.

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yea bpres 5 and 6's $3 a piece and lasted 5 some years isnt too bad I suppose if thats the case.

I just want to rule out everything that I can if its not too expensive. Only spent $100ish for everything.

I know that its an old motor and it will have its hiccups I just need it to last me until I can afford another car again. Its always treated me good and I know that I have not done any maintenance on it in a good while, it deserves the attention that it needs.


It does throw me off that it very well could be not even a miss at all. just an odd pop sound from the bay, don't really feel like a shake, rattle, or hesitation when it happens. Front end is almost entirely rebuilt about 2 years ago,(balljoints, link kit, shocks, tie rods) other than it needs an alignment its held up alright imo.

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If the intermitant pop Is random I would check the distributor for worn bushings.  Our race car drove us nuts for 2 weeks trying to find this problem.  If the distributor shaft wiggles re-bush.

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Unlikely, as the EI distributors from '78 on only spin a magnet. Earlier points shafts have to push sideways against the spring holding the points closed to open them. Shaft wear on a points distributor would affect the dwell so would also affect the timing. EI dwell is designed in and the firing is magnetically triggered.

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Alright so I was able to get the time finally to do the valve seals, spark plugs, cap and rotor today. Got it all back together with no issues and went to fire it up and got a backfire through the carb. cranked it 3-4 more times around same thing. checked sparkplug wires 3 times and still doing the same thing. pulled all the plugs and had good spark and not a fuel issue. any ideas? reinstalled the old rotor and no difference. I did take the rocker shaft off to get the springs off but put it back in the same orientation. set valve lash on everything and gapped plugs properly. Truck has never had an issue like this and I am at a standstill now.

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Firing order is counter clockwise. 1, 3, 4, 2.


Take valve cover off and re check all valve lash settings. One, an intake probably, is too tight. Should be 0.014"  0.012" on the Z24


Still got the old cap? try putting it back on.

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Read somewhere it was supposed to be .009?? and 6/8 of the valves were set at that before i had even messed with it. I had accidentally messed up on the front 2 so i'm not sure where they were at previously. at which I set at .009 (Note: on a cold engine) I never did try the old cap. When reading the cap I did get a bit confused on where the wires go due to the placements of the letters on the cap. I did move all the wires to the right and left of the letters and I'm pretty sure they are correct where they sit running counter-clockwise. 

I will try setting the valve lash tomorrow since I'm just mentally beat trying to figure it out today.

Was there any special way to set the valve seals? I just pulled the old ones off cautiously with channel-locks and installed new ones with a 14mm socket and small taps with a hammer. Only reason why I ask is because when cranking the motor with fuel pump disconnected I did have oil cap off and still felt some blowby.

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Cold setting would be about 0.002" less than the hot setting. It assumes that when heated the correct clearances are reached but, they could be set at freezing in a cold garage or 100F in a heat wave. They should always be checked when hot. There is a quick and easy way to do this before the engine cools off. Warm engine thoroughly 15-20 min. Remove valve cover and set engine to TDC compression on #1 cylinder. A quick look will confirm that both #1 valves are up and closed.


Set both #1 valves, intake on #2, exhaust on #3


Turn engine ONE revolution to TDC on the #1 exhaust stroke. A quick look will confirm that both valves are closed on the #4 cylinder.


Set exhaust on #2 and intake on #3. Set both valves on #4 cylinder.... done.


No repeated bumping the starter over and over and over. Should be able to do all 8 valves in well under 10 min 

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Starting to sound like my issue is I never set motor to TDC.. I just rotated motor until the cam lobe was facing straight down/across from the valve I was adjusting was closed and set lash... and them being set to .009 would make sense that its not opening properly because when cranking motor it sounds like its building an ass load of compression then it blows it out the intake.


if this works ill definitely warm motor up and check them at operating temp.


hoping now i didnt risk bending a valve by doing it incorrectly O.O

Edited by Toxicrain
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I guess I said 0.014" earlier. I meant to say 0.012" for both.


Turning the engine for each valve will work just takes longer. Even 0.009" will work and the valve will be closed but the valves are cooled only by their contact with the seat. For some reason Nissan set both at 0.012 " when hot.

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It seems like a dark fluid on the  walls of the carb by the throttle body. It looks like its blowing all the compression back because it blew one of the caps i had on a fitting on the intake after weber swap. Sucks that I have pretty much ran out of time on borrowing a vehicle and now stuck with searching for a new one by the end of the week.

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