4.0 Low Speed Misfire/Hesitation

Hi Rennlist, I am out of ideas. I have a low speed, intermittent misfire and hesitation on my 4.0. It is random and does not appear to be temperature specific (ambient and engine temp). The actual symptoms are hard cold start, hesitation and lumpy running when I barely get on the gas to take off. If I rev it past 2k I can take off with hesitation. I rebuilt to 4.0 using Nickies. It is otherwise stick aside from rods, UAOS and IMS Solution. Fuel trims are pulling fuel at both idle and high speed. I get occasional misfire codes on cylinder 2 and 6 as well as catalyst in efficiency or slow O2 sensors. here is what I have done so far (at rebuild 25k miles ago) New plugs/coils New upstream O2 sensors New MAF New CPS New intake crossover solenoid New Evap solenoid (old on was ok) FRA B1 .83. B2 .79 RKAT B1 1.5%. B2. 2.63% Cam dev B1 -.26. B2 +1.3 Fuel pressure 55psi at idle and hi rpm Cam advance tracks spec value on both banks. Active test on hi lift kills engine at idle and I can hear the solenoids clicking when engine is off. TB and pedal track to spec values. Intake, exhaust, smoke tested and is tight. I took the cats off and scoped. Both look perfect. I clamped the fuel evap line with no effect to engine. MAF reads 17 kg/hr when engine cold and 14-15 when hot. UAOS seems to be working properly. Main ground is clean and electrically checked. Positive line also electrically verified and visually verified. Alternator is charging correctly and battery voltage is correct. When I accelerate (increase combustion ch pressure) the fuel trims go to 1. As soon as steady state is reached (no matter rpm) the trims return to .83 and .79. It pulls like a freight train when I am on it but is undriveable The rest of the time. Any help is appreciated!

So when did the issue start ? Has it been doing this for 25k miles ? I think you should concentrate on the missfire codes, since that is the only codes you are getting / and the DME is detecting ...

PT3, It ran great in the beginning. This problem has been getting progressively worse. I pulled the injectors and bench tested to see if they had a �good� pattern. All appeared the same. The coils and plugs were replaced and did help but still a misfire. I am going to hook up Durametric and see if it is consistently those two cylinders misfiring. I don�t understand why the fuel trims go to 1 when on the gas (not even full throttle)

I am going to take a look at the valve high lift solenoid. This past week I drove the car at 3500+ rpm all the time. No hesitation or anything weird. Below 3200 it hesitates when getting on the throttle. At low engine speed it will hesitate and basically stall for a second while I get going. If the high lift solenoid is sticking or leaking maybe it does not go back to low lift at low rpm.

Some feedback from our digital overlords: https://share.gemini.google/SoNVH9Uk1WHS

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Thank you Mr. TwoBags! I have done most of what is on the list from the overlords. Last Saturday, I pulled each lift solenoid and both work smoothly and Ohm out as expected. I performed yet another smoke test on the manifold. While doing so, I noticed the UAOS diaphragm was oil stained a bit (this was new). I took it off and found that the hose from the catch can to diaphragm was not clamped well in the catch can side. So I squished the pinch clamp a bit more and that tightened it up. I then took the diaphragm apart to see how much oil was in there. Not a lot but I cleaned it and reassembled. I have only had water in my catch can in the winter, never any oil. I saw that there was oil in the hose from the diaphragm to the intake manifold, which I thought was odd. And unnerving. So I scoped the intake runners and found each bank had a small amount of oil (very small) sitting at the bottom of the valves. I never measured the crank case pressure but pulling the cap off seemed pretty easy as the suction was light. I wonder if I had a vacuum leak at the catch can and it not only caused the fuel trim issues but may also not have pulled enough vacuum on the crank case. The mechanism to allow oil past the rings and blow up into the intake is not something I can imagine. Maybe the valve guide seals are bad or missing (heads rebuilt at time of engine rebuild). I have not run the engine since I needed to reseal the fitting for the IMS solution that had started leaking. I will start it tonight and see how it goes.

Well, none of what I have done has helped. In fact, it got worse. I detected a cylinder dropping out intermittently on the freeway. Hesitation at step in is intermittently worse.

FRA B1 .83. B2 .79 RKAT B1 1.5%. B2. 2.63% Correct me if I'm wrong, but aren't these numbers indicating that you're adding fuel at idle as opposed to your stated pulling fuel?

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It took me a while to wrap my brain around what the FRA and RKAT numbers meant. FRA is out of 1.00. That is a perfect trim. If the DME is showing .80, that means it is pulling 20% fuel. If FRA was at 1.20, then the DME is adding 20% fuel. RKAT does show adding fuel. I misstated that in my post as I was generalizing based on long term trims.

So you don't have any fault codes?, and the DME is happy with all the values and self tests that it runs? Has anyone messed with the DME programming?

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This all started with upstream O2 codes. Naively, I replaced them to no improvement. That led to catalyst inefficiency codes. Fuel trims have been pretty steady at .83 b1 and .79 b2. Things got more interesting today. Drive it to work after resetting the fuel trims. Same hesitation after a few miles. Then it started running on 5 cylinders for a brief 20 seconds then went back to all 6. came out tonight to drive home and it was a longer than normal crank to start (which has been happening with the fuel trims where they are). Fired up and I noticed white steam from the right exhaust. Went out and it smelled of coolant. Then it stopped and didn�t come back. However the Coolant tank is nearly dry. Never overheated. Car runs like it has been with no abnormal sounds. It�s parked now until I can get to it this weekend. I don�t believe in coincidences but this one is baffling me.

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Surely with all that it set some fault codes right ??? The white steam , coolant smell, and dry reservoir is a dead giveaway that you have a cracked cylinder (or head)....along with running on 5 cylinders for a while.. Which cylinder had the misfire/code ? Start there looking ..

Sorry Skip, I neglected to mention I had misfires on cyl 2 and 6 and sometimes 2 and 6 a while back. Coils were cracked so I replaced them and that helped but did not alleviate the hesitation problem. It made the misfires less bad 😆 Here are the codes that I have been getting P0304 P0302 P2096 P2098 The codes are not as frequent and had not occurred in over a month until today. I cannot get a ready status for cats though. Tonight, the only code is P0302 for cyl 2 misfire. That coincides with the bank the vapor was coming from. This is a 4.0 built with Nickies and has 26k miles on it.

Shouldn't a 4.0L register a higher MAF value than 17 kg/hr, which is specified for a 3.4L?

Yeah always focus on the first code that pops up first, (first or worst ) the following codes can be collateral and not root cause . ( if it takes a while for them to pop ) So P0302 , check that one for the coolant intrusion

I will pull the plug this weekend. I don�t want to crank the engine in case that cylinder is slowly filling with coolant.

If there was coolant getting into a cylinder, I�ve always been told that particular spark plug would be a lot �cleaner� than the rest, as in steam cleaned. Are you able to do a leak down or compression test? I ran a stock tune on my 4.0 for a while and it ran fine. Any tapping sounds or indications that it runs on 5 cylinders at idle? Weak spring, stuck valve/lifter? Hopefully the coolant leak is a separate issue (cracked tank?).

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Not just the spark plug will be cleaner, but the whole combustion chamber/piston/head will be cleaner...........It one of the telltale signs of water/coolant intrusion ...........Back in the 80's I had a turbo-charged street car running 25psi with " water/meth Injection " and it kept the combustion chamber clean as a whistle ..

Well, not what I expected to find. Cylinder 1,2,3 plugs all look the same and looks like pre ignition to me. All three were finger tight so I suspect they were getting hot. Pistons all look the same are don�t show signs of coolant intrusion. I drove it home 20 miles from work with no issues and the coolant stayed at the same level. Is it possible to have an intermittent head gasket leak? I am going to bank 2 next. Cyl 1 on left

Coolant steam coming from one pipe will never cure itself..... I doubt it is a crack in the cylinder since you have upgraded Nikies in there, but a crack in the head, especially in the SAI port, can definably be logical...

Check this out, Skip. Bank 2 looks different, cleaner. I think cyl4,5 may be the culprit. Cyl 4 is on the right. Very clean plug. Cyl 4 Cyl 5 Cyl 6

Why are those camera pictures so "blue" looking ?

I think the pics are blue because they originate from a low priced borescope and then I am taking a picture of that display with my phone. I may need a higher quality borescope. I think I will drop the engine anyway since I know the bank 1 was pulling coolant. My concern is that the plugs were causing detonation and I may have caused damage to the piston/rings/bore. Won�t know till I get in there!

Get a better borescope and look inside the cilinders again before dropping the engine. The ones with remote swivelling camera have become affordable lately and allow you to look 'back' at combustion chamber and valves when entering through the sparkplug hole. Detonation will have left witness marks on the piston edge that you will see with a new borescope. How can the plugs cause detonation if they are of the correct heat range?

Hartailer, if the plugs are loose then they will not transfer combustion heat to the head. This causes them to get hot enough to cause pre ignition. I agree I need one of those articulating scopes. But in this case, the engine drank some coolant as well (though stopped somehow). So I need to pull the heads and inspect for damage.

I agree with Hardtrailer, I would do as much testing and gathering of information as possible before dropping then engine... Once you drop the engine you can no longer test the engine while running it if you think of something you wish you had done... When you have identified which cylinder has the coolant intrusion, and if the intrusion is from the intake side, the combustion chamber, or the exhaust/SAI side, then drop the engine for local/visual inspection.

Those are fair points that I will take. While I wait for the new scope I will perform a cooling system pressure test and see if I can find where it is intruding (with my crappy scope). Im not sure how to tell the difference between intake and exhaust side leaks. Remove the exhaust manifold?

I would gather information like does it leak worse hot or cold.......? Is the leak causing the missfire or are they two unrelated issues...? Is it worse after sitting from hot soak ? Going on a "Visual hunting expedition" to find a problem that you are not sure of what you are looking for, or have specific "bench tests" planned to help identify what you're looking for hardly ever works out.. I have seen WAY to many people go on this "visual hunting trip" only to not "see " anything and have to put it all back together just to get more info.....thats a lot of work and mental exercise waisted

Skip, you are making sense to me. I was concerned that running the engine may cause some damage, particularly at startup if I filled a cylinder with coolant. But the leak does not appear to be that bad considering it didn�t use any on the way home. I have a spare set of plugs that I will put in since the bank 1 plugs are pretty well cooked. I ordered a new scope and a pressure test kit too. I appreciate the guidance.

Installed the new plugs. Started up, no smoke but a lot of lifter noise on all cylinders for some reason. some misfires on 2,4,6 when I first started it. Once warmed cyl 2 was misfiring consistently. Short drive and it still has the hesitation. I blipped the throttle once and got a backfire through the intake. Right side exhaust tip appears to be gray now. Left side Right side

When you say cylinder 2 is misfiring consistently, do you mean it misses at idle , light load, heavy load, full throttle, at any and all RPMS low and high, hot or cold ??

I think you nay have two different things going on. The misfire at low rpm only could be an intake lifter stuck at high lift.

The missing on cyl 2 happens at idle, low rpm high rpm (no load). It seems to get worse when up to temp. I logged data while I drove it today but have not figured out how to retrieve the data so am reading the Durametric help files. I want to see if it misses at high load. I had a Trail 70 when I was a kid and it developed a hesitation just off throttle. I foam my figured out it was a sticking intake valve. I readjusted the clearance and it ran great. Like Thepf mentioned, maybe I have a sticky valve lifter. And a blown head gasket.

Dealing with cylinder 2 misfire as well, but mine only does it on high load right after 5.2k RPM CEL flashes a few times then goes away. Sounds like mine might be a sticking valve lifter as well.

Been busy at my house so not much diagnosis done yet. I now have a high quality, articulating bore scope and a coolant leak test kit. This weekend is occupied with moving my sone back to ID for his first job after graduating in May. I will dive back into this the following weekend and see what turns up.

I had some time today and read through the shirt Durametric manual. Apparently the Worl Log does not collect data and I do not see any way to do that. Next week I will take a spin on a country road and see if I can see the high rom cyl 2 misfires. If there are not any, I think I may have a bad intake lifter or two as well as a head gasket. I started it today to get it out of the garage and it was missing again in Bank 1. After a few minutes of idling it blew steam out the right tail pipe for about a minute. Then stopped. Still running very rough.

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5.2k RPM is when it comes off of Variocam, if you have a 5 chain engine. May be related to that.

Head gaskets never fail in these motors but u do have a rebuild so maybe?

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Agreed. I�m wondering if I torqued the heads improperly when I assembled the motor. The APR instructions for the studs I used don�t follow the Porsche/Raby instructions. I would rather a head gasket over the alternatives. And since the engine has never been overheated the chances of a crack seem low.

Removed cylinder 1 and 2 plugs. Both were black but not sooty. Used the new scope and see that cylinder 2 is cleaner than 1, like it has had steam through it. I don�t see anything unusual with the valves. Pressure test of cooling system didn�t yield anything. It holds pressure and I don�t see any coolant seeping into the cylinder. What ever the issue it is getting worse. The engine runs rough and misfires worse than before. All in cylinder 2 at low rpm. Misfires reduce substantially at 3500+ rpm. Cylinder 2 Cylinder 2 Cylinder 1

🤔 looks to me like a piece of the piston crown is missing?! Hope I�m wrong. have you done a compression test?

I questioned that also. It looks like part of the valve reliefs to me. I have not done a compression test but it cranks evenly.

I didn't read every post; have you done a test for combustion gases in the coolant?

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No but that is a good idea.

Update: had some time to start working on this thing again. The leak down test showed air out the exhaust and the coolant reservoir on cylinders 2 and 3. At this point I decided to drop the engine. Got the engine on the stand and removed the exhaust. When I rotated the engine about a cut of coolant came from bank 1 exh manifold. Removed the head and found coolant in # 1 and 2. The intake valves have a white coating on them. The exhaust ports are black and gooey. I did not see any evidence of the head gasket leaking and the heads studs were all to proper torque. I also noticed wetness on # 1 and 2 air injection ports. sure seems like a cracked head. Dunno how that happened since the engine has never been hot and Hoffman pressure tested them when I built to 4l. I am going to reach out to Hoffman for next steps. I could not find any �bad� intake lifters. I followed Raby�s procedure for inspecting for cracks and binding. However, I don�t know that that is a conclusive test. Pics below. #1 Moisture in the air injection groove at 1 and 2.

Mate I have been following and thats a tough break!!!

I talked to Len Hoffman and he suspects with high probability that the head is cracked in the secondary air passage. This would be confirmed with a pressure test. Either way, the head cannot be repaired since that area is not accessible to weld. I�m in the market for bank 1 head. No cracks, please.

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Sorry you're going thru this after your proper rebuild. But at least you've now found the problem. Can't the secondary air passage be blocked off and even SAI delete done if wanted...?

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Thanks for the sentiment, @allcool I am sending the head to Hoffman to pressure check and salvage parts for the next head. The issue is that the crack is between the air passage and the exhaust port. Blocking the SAI would not keep coolant from entering the port. Plus I live in the �Republic� of CA and the smog demons will not allow me registration without the secondary air installed.