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Jeep cuts out under constant acceleration

2K views 30 replies 8 participants last post by  NittanyGen 
#1 ·
1985 CJ7 4.2L

So I ran into some weird stuff today while driving. My DD is down with a cracked rim, so I'm stuck with the Jeep to get back and forth to work until next week, so hopefully I can get this ironed out tonight.

Here's what happens. I notice it in 4th gear or 5th gear under steady, constant speed. I can't tell you the mph because I don't trust my speedometer, but it happens around 1500 RPM in 5th gear and 2000 in 4th. While all seems good and well, I will be driving along and then it feels like someone has applied the brakes and the Jeep begins to decelerate. It doesn't die or turn off; I can put the clutch in and coast for a few seconds and it usually returns to normal, however, the longer I drive, the worse it gets. When I have the jeep sitting in my driveway, idling, it idles smooth and confident. However, when I approach a stop after this weird cut off scenario, it feels like it's misfiring all over the place. When I was driving down my street to get home just now, it happened, so I tried to downshift to 3rd and put my foot in it and see what happens, and that was the only time it died. But it started right back up while I was rolling. It feels like it's flooding out, as opposed to being starved for fuel, but if I adjust the idle mix screws anymore in, it begins to run like a tractor engine. I thought maybe the float was bad, but it floats nicely and is adjusted according to the rebuild kit instructions. I haven't tried replacing the needle yet. I'm kind of at a loss here fellas.

I've done the Teamrush upgrade and Nutter Bypass. Fuel pump and fuel filter are also new. I replaced the fuel lines from the pump to the carburetor, but everything else is original. I originally thought maybe I was getting starved for fuel, but it's not at high RPM, and it doesn't die, it just doesn't respond to throttle anymore. So that scenario seems less likely. I also don't want to condemn the ICM because the engine only died when I tried to force it through the hiccup. If I put the clutch in and let off, it gets back to normal and I can drive for about 30 more seconds before it happens again.
 
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#5 ·
Is the fuel tank vented properly?
Leave the cap off and see if it runs better.

Best to eliminate the simple stuff
I can try that on my way to work tomorrow. Would that possibly help it at idle too? I notice it has a random little hiccup if I set idle to around 650rpm. If I bump it to 800ish, it lessens, but I can still hear it. The hiccup/misfire gets really awful after this no-power scenario occurs.

Sent from my SAMSUNG-SM-G891A using Tapatalk
 
#7 ·
Sounds like the ole water in the fuel routine. Try a bottle or two of gas dry and a fresh top off of the tank. Water sinks to the bottom of the tank, when you crank it up in the morning, it gets sucked up the fuel line. Our weather has been odd, cool mornings and warm days, I'm near Beaumont, Texas so I know yours had been like ours. Friend had a small gas container in the back of his truck the other morning, had hardly any fuel in it. Its was sucked in so tight the other morning I thought it was going to crack or split. He had used it the day before to mow and closed it up. By noon it was swollen out like it was about to pop. So the weather will make the tank breath and when it does, our moist air will cause problems.
 
#8 ·
We have had some pretty strange weather, haven't we? I went a picked up a bottle of Heet this morning and filled the tank. Yesterday it malfunctioned on a 2.5 mile stretch where I coasted around 1500 RPM in 5th. Today, it was fine. I'll drive it longer this evening when I leave work. Interestingly enough, I thought we had 15gallon tanks. When I filled up this morning I put about 17 gallons in. Also learned that my fuel level sender is accurate, unlike my speedometer. Funny thing about the speedo is it will stop moving at 15mph, until I let off the gas and coast, then it goes up to about 40mph.

What do the plugs look like?

Matt
The plugs looked pretty good in my opinion. I misplaced my gap tool, so I couldn't confirm the gap, but they didn't have any deposits or anything on them. A little carbon around the base, but the electrode looked good.
 
#9 ·
I also heard a hissing noise when I was looking aroudn under the hood. I ended up following it to the vapor canister. When I squeezed a hose that leads up and into the back lower part of the carb, the hissing stopped, but it also started to idle poorly. Didn't die, but weird idle. Should I be concerned about that?
 
#11 ·
Yeah, I have replaced a few here and tehre when they are stiff or break. I haven't replaced the ones going to the canister yet. I replaced the ones going into the carburetor from the CTO. All I have left are the big hoses that go to the canister. I can likely get some hose from work. I'm going to run through the FSM today and see if I can get some diameters so I don't have to buy a whole bunch of stuff and try them one by one.

From what I'm reading, if the vapor canister malfunctions, it can cause an issue similar to what I'm struggling with. First things first, replace the hoses leading to and from it and replace the filter inside it I suppose.
 
#15 ·
I found one website that claimed they had it. $30 is worth a shot to see if they do, I'll probably get a cancellation email tomorrow after it processes. I've found a few others that look similar to the one in that thread as well as the one in the retrofit thread here on JF, but they're almost $140 each. I'm going to keep looking around and probably scour ebay and craigslist later and see what I can drum up.
 
#19 ·
Wanted to update my thread and let future people know that it seems I have remedied my issue by adding dedicated grounds. I changed the ground point for the block and the negative ground cable and added a dedicated ground from the cylinder head. I also added an extra ground for the distributor that the teamrush upgrade suggests, but that didn't seem to do much, if anything. By adding the ground to the cylinder head and updating the block ground, the ignition coil no longer gets so hot that I can cook on it. Nor does the jeep fall on her face anymore. She still has a slight hiccup at idle every few seconds or so, but I think that may be the result of an exhaust leak.
 
#20 ·
I am having the same symptoms on mine and I swear it is a manifold leak but it has been checked for such and the cause can't be found. Racking my brain trying to find the reason its doing this. Everything else related has been replaced with NEW. So, carb is new/rebuilt, EGR, fuel filters, various vacuum lines, re-did the emissions canister and such. Still has a slight hiccup. It appears to go away when it warms up so this is why I am leaning toward a manifold gasket issue. I should just replace that manifold gasket just for peace of mind. I bought the darn gasket but when i had it tested for final smog pass, I was told it was fine so never thought twice about replacing it. The plugs look great but I bought some new replacement Champions anyway. The spark plug wire closest to the firewall was bent at the plug terminal. It was like a 90 degree bend. Never saw this before? I thought it might have been this. I guess the smog tech shoved it in there so hard it bent it but I have since checked and pulled them all and they are all straight now. Still wondering now if they are internally damaged by him pulling on them real hard. I need to buy another good set before I put in the new plugs so I know the wiring is eliminated as the cause. So, let me know what YOU find is your cause and I'll see if its been overlooked on my end. Good Luck

P.S. If you get vacuum hose diameters for each vacuum line size, post them. I too would like to just buy them all at once and be done with them. I'm just not sure what sizes they are because they are not marked and some look swollen from age.
 
#21 ·
I'd be glad to share the vac line sizes, I'm just not sure what you need. I only have about 5 lines left now. While upgrading the ground solved my overheating coil, it didn't fully solve my surge under load. It happened again yesterday. I'm starting to believe I need to really fine tune my carb. I'm thinking a jet change is in order.
 
#22 ·
I did a swap with a complete rebuild and it was awesome. FYI, my accelerator pump on the old carb was bad. The rubber had deteriorated causing it to flood within. I re-did my jets as well prior to smog testing and I got called on it by the state tech. Funny thing, he knew exactly that my jets were bored, lol. So, I just replaced it with a whole new carb. I know on mine it is something simple but so simple I am over-looking it. I am going to check the wires again tomorrow, look thoroughly for an exhaust leak and check the rest of my lines. I feel for you because it drives me nuts. It runs great when warmed up.
 
#27 ·
Just wanted to add an update for anyone using the google machine that may have stumbled upon my thread, the items I have done so far has lessened, but not eliminated the issue. It seems to worsen with ambient temp differences, but I've yet to fully pinpoint the issue. I have a little bit of walking around money so I've been tempted to go EFI, but I'm 99.9% convinced it's not a carb tuning issue. especially since it's a brand new redline weber 38. I probably don't have it tuned 100% because I'm a huge newbie to carbs, but I know I have it tuned well enough to get me by. Maybe not pass emissions and run the most economically, but well enough to not expect to have a stall scenario.

I have yet to drop the fuel tank and replace any fuel lines or anything in the back of the truck yet. The PO (my dad) had said the tank had never been pulled so I knew that if I dropped it, anything rubber was going to need to be replaced as I would probably damage it just taking it apart. But that's when it clicked! At least I think it clicked. Since my issues don't arise at idle, and they don't arise under acceleration, but rather at cruising speed, I think I'm losing head pressure. I'm guessing I either have an issue sucking in air on the feed or I have a blockage on the return. This would also explain why I have a hard start after it sits for a day or so. Even if I pump the pedal a few times, it still has to turn over for awhile before it starts.

So, unless y'all tell me otherwise, I'm going to drop the fuel tank, replace anything and everything rubber, as well as pull the fuel sending unit and replace it. In the meantime I'll check to make sure there's no trash in the tank. I think the tank is the 20 gallon plastic tank, but I guess I'll find out. One thing I haven't decided if it's worth the investment yet is if I should replace all the metal fuel lines, both feed and return, as well.
 
#30 ·
Update: Today was the day I tackled dropping the fuel tank. Sorry it has been almost a year since I last posted, but with school, kids and a new job I've been busy! I am pleased to say, I have finally graduated, so now I have plenty of free time! I did not get a chance to drive it, but I will say that after putting 2 gallons of gas in it, and cranking it over. It started faster than it has ever started since I've had it. I look forward to driving it tomorrow to see if that issue is fixed. If it is, I can finally move forward with the project. I put everything on hold since it was pointless to spend money on it if it wouldn't run right.

This is what the sock looked like. All the fuel lines at the tank had the stupid crimped hose clamps, so to the best of my knowledge, this sending unit/sock is original 33 years old.



The tank looked good, some sediment at the bottom, but I washed it out and drained it. As it currently sits, the only thing that I haven't replaced on this system are the metal fuel lines, the gas cap, and the vapor return line. Fingers crossed that it works. My theory is that the sock being that tarnished and pinched at the pickup was starving the carb. Which means I probably need to retune it as well; so I'll do that before my test drive tomorrow. But I have high hopes!
 
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