Running poorly/misfiring, code 33

**1project2many** · 07-26-2023

Hey, nice to see you here.

MAP sensor high indicates the ecm is reading high voltage on the MAP signal line during specific operating conditions. For default configuration, here's the code definition:
33 MAP sensor signal voltage was too high (low vacuum) for 1 second when throttle opening was under 20% and engine speed was above 1250 RPM.

You should be able to see MAP voltage on the scantool in key on / engine off condition to compare the reading obtained at the PCM wiring harness connector using a DVOM. Additionally, you can create simple resistor networks to connect between 5V, signal, and ground to create a "between" voltage to send to the pcm.

In a stock wiring harness the 5V circuit and GND circuit for MAP are shared with the EGR valve. Any issue which can cause a short to 12V on GND or 5V REF or an open on the ground circuit will also raise voltage on the signal line as seen by the ECM. To catch this on the DVOM you should be measuring voltage between the sensor ground and the sensor signal line.

Since the engine reacts badly while wiggling the harness, I would likely try connecting the laptop to watch ALDL data then taking hold of the harness and performing a Ford "Wiggle Test." Watch for significant changes in MAP or another sensor reading on the laptop.

I have had troubles years ago with intermittent code 33 on TBI equipped 1995 vans. In one case I found a broken wire in the harness causing an intermittent open. It was only apparent when the engine was very warm. In another case an aftermarket EGR valve caused an issue. In both cases it was frustrating to find because there were no symptoms showing while the van was in the shop.

Keep us updated!

**JeepsAndGuns** · 08-02-2023

Thanks for the tips.
The jeep does not have a EGR and all the related wiring was removed from the harness long before I installed it years ago.

I forgot to include it in my original post, but one of the times I had the laptop hooked up, I applied vac to the map sensor and had my wife watch the screen while I moved and wiggled the harness under the hood. She said it never moved.

I have had wires do strange things in the equipment I work on over the years (I am a small engine mechanic). Lots of intermittent issues and they are always the hardest to trace down.

My local junkyard is about a hour away, but I have a gun show to go to in a couple weeks in the same city it is in, so I plan on stopping by there and picking up a pigtail for a ecu so I can have some spare pins.
I will then make and run a new temporary wire from the map to the ecu and see what happens.

**tplasek** · 08-03-2023

You can find the TBI ECU pins on Ebay. I bought a pack of 15 for around $10.

Someone also previously mentioned that you need to use the sensor ground when measuring the voltage on signal or sensor voltage. This is important as this is the voltage the ECU sees. DO no use a chassis ground.

**JeepsAndGuns** · 09-05-2023

So I got a ecm pigtail at the junkyard and finally had some time to try and fool with it today. I have just been super busy lately.

I made me a new wire for the MAP signal. I just laid the new wire on top of the harness as it was just temporary.

I still could not get any reading on the datastream of the map sensor with the engine not running. Everything else seemed to read ok.
I started it and it still ran like crap, I had to keep it running with the throttle.
I did get map readings with the engine running. About 70-80 at idle, and it would go up if I opened the throttle.
After keeping it running for a few min, it would idle on it's own, but barely, and still ran like crap. Then after idle for a short bit, it would stall.
I had to run into town to do a couple things and finally found a vac gauge at the parts store.
I disconnect the vac line going to the charcoal canister purge and hook it up there.
At idle, it is around 5, maybe a little more in-hg. When I open the throttle a little, and the datastream shows 50 kpa, the vac gauge read about 15 in-hg.
Vac seems to go up when I open the throttle, then drops down at idle.
However, this engine has never really had high vac at idle.
This is the camshaft I have in it: https://www.compcams.com/xtreme-ener...c-290-401.html
I figure since it was made for carbs, and having 110 LSA is probably the reason for the generally low vac at idle.
However it has never been a issue before and it has been great and trouble free for years after I tuned it.

I will attach two log files.
One was from several years ago. I have no idea what I was doing, but it appears I made a short trip down the road and back. You can see what everything read while it was running normally.
Then I will attach the file I recorded this afternoon.
I started the recording, waited a couple seconds then started it. You can see from the tps readings, I was having to keep the throttle open to keep it running. I did let it idle for a little bit and it roughly did, then I opened the throttle some for a bit then let it idle again. It did, but still poorly, and then it stalled. I waited a couple seconds then stopped recording.
It looked like the map went real high as it shut off. I do not know if it was just the result of the engine stalling, or it was the reason the engine stalled. I tried watching it a couple times but do not see one thing standing out at me.

The files are the dates I made them.
$0D, 16197427 ecm converted to MPFI.
Jeep/AMC 401

I removed the new wire and hooked the old one back up and it did not really seem to change anything.
I have also cut all the zip ties and pulled off almost all the wire loom to inspect the wiring and have yet to find any spots with issues.
I double checked I was getting 5v to the map sensor, ground, and when using a hand pump, map sensor voltage lowers as vac goes up.

**1project2many** · 09-05-2023

It sounds like the MAP readings when the engine is running are pretty close to accurate. I think I would move away from trying to get MAP readings to display on the laptop during KOEO and move toward diagnosing the driveability issue. MAP codes can be set by a very poorly running engine.

Assuming the spark is working and the timing is close, I believe the next test I would make is to ensure all eight injectors are firing. Injectors not delivering fuel can seem very much like a spark issue. I would probably start with a noid light at each injector connector, cranking the engine and watching for the light to blink consistently. Then I might do my best to get the engine to start and I would use a screwdriver to "listen" to each injector click or use a special tool that indicates when an injector is clicking to try to determine if there's a problem with injectors.

**TriumphR3** · 09-05-2023

I remember one time I had a contact in the connector on the ecm that went to one of the injectors that became intermittent. Vehicle ran very poorly. It was very difficult to determine, but I finally found it.

**JeepsAndGuns** · 09-06-2023

Originally Posted by 1project2many

It sounds like the MAP readings when the engine is running are pretty close to accurate. I think I would move away from trying to get MAP readings to display on the laptop during KOEO and move toward diagnosing the driveability issue. MAP codes can be set by a very poorly running engine.

Assuming the spark is working and the timing is close, I believe the next test I would make is to ensure all eight injectors are firing. Injectors not delivering fuel can seem very much like a spark issue. I would probably start with a noid light at each injector connector, cranking the engine and watching for the light to blink consistently. Then I might do my best to get the engine to start and I would use a screwdriver to "listen" to each injector click or use a special tool that indicates when an injector is clicking to try to determine if there's a problem with injectors.

Would have not thought to check the injectors. They were new when I put the MPFI system in. Bought them as "new take off" that had been removed from a new crate engine. They did in fact look new and un used.
I might have in total, less than 10k miles on it over the years since I built the system. I know that means nothing as even brand new parts can go bad.
Of course it might not be them and could be the wiring.
I have a set of noid lights somewhere, just gotta dig through my storage to find them. Not even sure if they have ever been used!
Checking the fuel pressure was one of the first things I did when it first started acting up. It was good and steady.

I will start digging into that and start checking things (weather permitting) this weekend. The weather has been real hit and miss here, and I do not have a garage or car port. So I am at the mercy of the weather.

**JeepsAndGuns** · 09-10-2023

I had some time this morning to work on it.
I dug around and found my noid lights. I also found a spare injector pigtail.
Before I started the engine or did anything, I unplugged all the injectors and using the spare pigtail, I added some lengths of wire to it and did a quick quick check on the injectors.
I tapped the wire to the battery really fast to simulate firing. They all clicked with no issue.
I did not remove them and they did not have any pressure, so I can not comment on the spray pattern.

I fired it off and one at a time, I unplugged a injector and plugged in the noid light.
The engine seemed to make a small noticeable difference when I unplugged each injector. Every few seconds I had to reach over and hit the throttle to keep it from dying.
The light appeared to be good/normal on all cylinders. No breaking up, no going out, and no issues I could tell. It looked the same on every one.

I tried again to check/set the base timing, but it will not run on the base timing. It either will start for a second or two then shut off, or just not start at all.
So with it running on timing control, I decided to give the distributor a little turn, maybe 1 degree.
It seemed to improve the idle, and it stayed running on its own. It still did not run great, but at least now it would not die.
I tried again to run it on base timing with no luck. Every time I try to run it on base timing, after I rest the ecu, I have to hold the gas pedal to the floor to go into clear flood mode to get it to start.
Just for the heck of it, I cranked it back up and pointed the timing light at the marks. My timing light has the dial on the back you can set to whatever degrees you want, and it will make the timing mark like up with tdc.
On this engine, it is a royal pain to get to and see the timing marks because of the design of the alternator and ac compressor brackets.
But from what I could see, the timing mark was hovering around zero degrees or just a tad retarded with the timing light set to zero.
I thought this was odd, as it should be around 24 or so degrees.
So just just for the heck of it, I turn the distributor a little more. The timing starts moving advanced.
The engine starts running a little better and idle improves. It now will throttle up and down almost normally. Idle almost sounds normal.
In total, I feel like I turned it almost enough to make me think it was one tooth off.
I marked the dist body and the rotor position, and I swear I got it back in the right spot, but anything is possible.
I tried again to run on base timing so I can check it and still it will not run on it. If I unplug the timing wire, it will not start at all. If I unplug it while running, it dies instantly.
With it running now, idle and reving up seem almost normal, but I am not going to try and drive it until I know the base timing it set right.
It is however, running very rich. At idle, it is running 11.5-12.5 on the wideband. I did not have the laptop hooked up to it to see any data.

The engine and the outside temp was hot, so I called it a day as I had a lot of other stuff I needed to do.

Hopefully next weekend (weather permitting) I am going to pull the #1 plug and make sure I have it at TDC (using a screwdriver) then verify my timing mark on the balancer, then see where the rotor is pointing.
Depending on all that, I may have to pull the dist and adjust it. I can not turn it any more as I a about to hit my power steering pump. (is a large cap HEI)