Misfire... help!

[vwtipeii]

Ok. I give up. I thought i had this licked up apparently not. Some basics:

Engine:
4.8L 505 Performance stroker
99+ intake manifold
62mm Jeeper Creeper TB
New TPS, New CPS, New plugs, Cap & Rotor, New coil. Wires are 2 years old and high performance "live wires"
Fuel Injectors are 24lb. Not sure of the brand and they are a year old.
Borla Stainless Steel Header

Symptoms:
Intermittently when driving on the highway at speed generally after an hour a misfire will happen for a fraction of a second. You can feel the whole Jeep lose power. Then as quickly as it happen it goes away then the Jeep is fine again. Then it happens again for longer and more often until eventually it is misfiring so badly that and continuously that the jeep looses power and I have to pull over. I have an air fuel gauge and it always shows the engine leaning out as the misfire happens. Codes thrown last night are/were 23 & 27:

23: Intake air temperature sensor input above or below acceptable voltage.
27: Injector #3, and/or 4, and/or 5, and/or 6 output driver does not respond properly to the control signal.

Last time this happened on the way home from KOH I pulled over and let her sit for a few min and tried starting her up again several times and the problem still persisted. I limped back to Brothers 4x4 and parked the Jeep in front of the shop and swapped my gear from my Jeep to another vehicle. By the time we swapped gear and I started the Jeep to show Haik the problem was gone and the Jeep was running like new again. Haik tried to drive the Jeep daily to replicate the problem however I think he never just drove it long enough to where the engine got hot enough to cause the misfire again. It never happens around town...only after being on the highway for an hour at speed around 2500 rpm or so.

I have another ECU that I have swapped in the past and the problem didn't clear and showed the same codes when I started the Jeep. I haven't started her since I made it home last night after sitting for about 30 min. She misfired almost the whole way home intermittently but I made it home. I have to get this fixed. I should be able to drive my Jeep across the country with no issues like this. Any thoughts or ideas are welcome.

Thanks,
Brett

[ROBERTK]

Where is the ECU located?

24lb for that sized engine (and depending on how they set internals, as well as compression ratio and cam) might be small at your stock fuel PSI, expecially with that size TB.

What A/F "gauge" are you running? Is it a true "wide band" or one of those pep boys specials that is very generic in reading? Where is the O/2 located for the gauge reading?

If I recall correctly your only at 31-35psi with the stock YJ fuel system. Thats not enough PSI with 24lb injectors I feel. A injectors job is to also atomize the fuel in to the air stream, and the more it does so the better and faster burn typically. The lower the PSI to the inj the less chance it has to do this completely or well.

On my stroker I run a 21lbs inj at 51-56psi. It is built to a bit more exact and critical level I am sure, with larger valves and alot more head work, but still only a 4.7'ish. So fuel needs are very similar. I never get lower than 12.5 a/f with my combo. I have ran all of Blue water desert challenge, and glen helan as well as many other trails in the last 2 yrs without an issue or any lean issue. SO I feel My combo is right for my needs. And might be something to look into on your stroker fuel needs also?

Easiest way and best way to achieve this is with a bypass fuel system that ditches the stock pump/reg, and do a ext pump on frame rail, and bypass reg in engine comp. area away from heat. That way you can put a gauge in reg and monitor it and bypass alot of the fuel up to the eng area back to tank and keep the pump and fuel cooler.

Theres some things to consider.

Good luck with it.

[Jason]

Brett, this might be a long shot but look for a grounding issue. You have enough lights and such to power the world, maybe a lost connection somewhere?

[vwtipeii]

I spoke to the engine builder today. Walked through the main issues. He too thinks it may be a ground issue somewhere. He said to go through the grounds and then if that still doesn't do it... to put in a stand alone engine wiring harness like Rob mentioned. That is another long and costly process however is probably what i should have done when I put the stroker in in the first place.

[ROBERTK]

Hotwire auto will take your harness and either replicate it with new or strip out what you dont need and send back etc.

Or this is a better but costly route.
http://www.naxja.org/forum/showthread.php?t=1110853

I hate to even pass that on, since thats what I Should be running and Extremely hard to find these days and brand new...

[RAT]

Brett,
My 2 cents probably aren't worth 2 cents, but a ground issue is usually not intermittent and usually not tied to temperature. Not saying it can't be, but not the norm. I think Robert may be on to something with the fuel supply. If you are leaning out the injectors the engine will get hot and set off the temp sensors. Check the plugs for lean out. Do you have a coolant temp gauge for the engine? If so is it high when the misfire starts? I would definitely rule out fuel supply issues before dumping money in all that wiring.

[aw12345]

It's not all that hard to rule out a fuel supply problem, which in this case would be a good starting point.
A fuel pressure gauge and an old scanner with 3 in one software for late eighties to early nineties cars will do the trick. Look at fuel pressure underload and fuel trim when it acts up and it will show if that is where the problem lies or you can rule that out as the cause of your grief.
Other than that a better mechanic wouldn't hurt either. Your Jalopie doesn't need a rocket scientist just a good mechanic to solve the problem

[6spdYJ]

Have you replaced the fuel filter? Also, there is a sock over the fuel pump pickup in the tank that can collect gunk as the pump runs. Once the pump is off, the crud falls down and she'll run again. Being that your rig's had 20 years to accumulate junk in the tank, it's worth a shot. The lean theory is interesting, but check to see if it's a time running vs. engine temp issue.

[Lumpy Grits]

My SWAG is fuel PSI issue.
Is the pump in the tank?
LG

[Russ Chung]

Brett,

I have a friend who owns a 96 ZJ and he had identical symptoms. The engine would run fine but after an hour at freeway speed it would lose power until he would have the accelerator floored and it would still lose speed. His mechanic put a pressure gauge on the fuel line and determined that his fuel pump would fail to maintain pressure after about an hour of driving at freeway speed. He replaced the fuel pump and that solved his problem. YMMV.

Russ

[Zoobi]

Here's my SWAG

I have to agree with everyone on the fuel issue but I think that is only part of the problem. The part that needs to be addressed also are the trouble codes and what they may have in common. Robert has some good info as well as running a similar system.

Code 27. injector does not respond. Of course jeep gives you very little info on how the pcm determines this. If the fuel flow is low and the injectors are running wide open, are the injectors having an issue over time or is the o2 sensor guessing based on what it sees. I'm wondering why there isn't a running lean code along with this as well as detonation on wide open throttle . If that dead ends then we look at how the injectors fire. The pcm powers the injectors and pulse's them based on input from several sensors one of which is the air intake sensor. Is the pcm getting a proper power feed as well as a ground? Everyone checks grounds (which is important) but over looks power. A poor power or ground feed will cause a drop in voltage. This will cause the connection in question to heat up and get worse. Considering the power draw of the injectors, they will strain a poor connection over time causing all kinds of weird problems as well as possibly this code.

Code 23. Intake air temperature sensor input. This sensor is mounted on the intake manifold in the air stream. It reads air temp and helps the pcm to determine how much fuel to deliver. Ive seen these go out of spec and make a vehicle run real rich or real lean. Based on where its mounted, at freeway speeds an overheating engine will have little to no effect here. This unit could be causing part of the problem but not all. I don't see how this would cause the code 27 to pop up. What they do have in common is the pcm and the pcm power and ground feeds.

My biggest concern is the fact it's intermittent. That normally points to a connection or a module heating up. I would think that a fuel issue would show as soon as you pulled a grade or went wide open throttle on the freeway. Weak fuel pump or poor connections to the fuel pump could be intermittent but normally get worse over time.

How I would attack this would be.
Check power and ground feeds to pcm. Include items that take power from the same feed. A volt drop test under a load is your most accurate.
Make sure the intake sensor is in good shape and good quality, check sensor harness and plug.
Find out what fuel pressure and injectors you need to run with that engine.
Find out exactly what temp your engine is running.
Make sure your fuel pump and connectors are in good shape.

What you need to do is list all possible issues. then positively eliminate each item.



Scott

[6spdYJ]

Looking at Zoobi's reply got me thinking toward the pickup module in the distributor. On the YJ it's used to set the timing for the opening of the injectors. Since it's lower on the block and essentially mounted in a stem away from the block it could take a while to get warm and go wonky. If the injectors don't fire at the right time... Just a thought.

[Zoobi]

Is the ignition module in those used as a cam sensor or just to fire the coil? If its used for injector timing, that has to be added. Either way those can cause some real odd symptoms especially if it's a low quality part.
Just recently I had my butt handed to me over 2 O2 sensors, a distributor (with cam sensor) and a crank sensor. All bought from auto zone at the same time and all failed around the same time intermittently.


Scott

[6spdYJ]

The ignition coil is controlled by the crankshaft position sensor. The camshaft sensor is mounted inside the distributor. The nice thing about OBD1 is that there is no after-cat O2 sensor.

[vwtipeii]

All great feedback and ideas. I dropped the YJ off at Hermosa Auto two blocks from my house. They were recommended by Tom Severin as I don't really have to tools, skills and time to rip into my YJ parked on the street. They found two or three of the injector wires in the main harness were not staying connected fully to the injectors. After some zip ties and a few other little things my YJ ran great all yesterday. I drove her for about three hours and did great... no check engine lights so far! I am still a little hesitant to say she is "good" from now one but as most of yall thought.. it was just a little thing. If i could get a brand new engine harness the replaces the stock one... that will probably be a great investment in the future.