75kona

Over the weekend I did an engine swap in my project jeep. First motor got nuclear hot from by the PO and grenaded a piston. Have the new motor in place and everything hooked up. When I try to start it, it will only run on wide open throttle but with very little RPM’s and is also pouring fuel in, you can smell it. My first thought is Crank Position sensor. Figure maybe we damaged it when taking the old motor our or stabbing the new motor in. I was not so graceful. Any other ideas as to the cause? When running it sounds like its hitting on ever cylinder, sounds like it has decent compression, and no odd noises so that’s good. Any help would be great!!

tjwalker

*Test fuel pressure. You should have 49 psi, plus or minus 5. The fact that you can smell fuel indicates it's time to get some fuel pressure data.

*You have a 97, what year engine did you swap into that 97 XJ?

*Use a code reader or scan tool to see if there are any active, stored, or pending codes.

Love that hoist you have there! Good luck and keep us updated.

75kona

What do you recommend for a cheap fuel pressure tester? When I depress the relieve valve on the fuel rail is shoots with some force which to me indicates it’s got plenty of fuel pressure.

The motor is from a junk yard and supposedly out of a 1997 with 140k miles on it. would a motor from another year be an issue? I thought they were interchangeable.

I was under the impression that it wouldn’t be throwing codes since it hasn’t really ran and had the chance to throw them. Am I completely out to lunch on this thought?

And thanks! I got the chain fall from my father and it has been put to work since I acquired my shop!

Thanks for the help! I’m not oblivious to my ignorance and appreciate guidance.

coreyt

I second looking into fuel, my guess your CPS would be working all or none. what about your Idle control valve? could at least be part of your problem

75kona

Thanks! The same intake that came off the old running engine (minus one cylinder) is now on the new motor. All the accessories off the old engine are on the new one as well. I figured the only think that could have changed was me bumping and breaking the Crank Sensor while I was wrestling the old motor out and the new motor in… I was not very gentle

Think I’m leaning towards buying a scanner before I just start throwing a ton of parts at the problem. Although the Crank Sensor is already on order and will be here this week.

tjwalker

According to your initial post, the new engine did run for a while, which may just be long enough to trip a code. Best off to take a look in case it did. Takes all of 2 minutes with either a scan tool or a simple code reader. This is a process of elimination and checking codes is job #1.

Pressing schrader valve isn't the best measure of fuel pressure. Yup, tells you you have pressure but just how much? You can often rent a fuel pressure gauge inexpensively from your local parts store, call around. It is actually possible to have too much pressure too, from something like a ruptured fuel pressure regulator. Gotta get hard data to prevent you from chasing your tail. Adopt a testing based strategy here as much as possible to determine what the root cause is. With an engine swap, it's possible that you have more than one issue; most of the time (check that; every time) I performed an engine swap, I had multiple issues!

Hope you ordered a MOPAR crank sensor. Many of the aftermarket crank sensors are total crap.

BlueRidgeMark

I think Autozone and Advance both rent them for free. You pay full price, bring it back within 90 days (I think) and get a full refund.

You could have 15 psi on that rail and still get fuel to spray out.

jordan96xj

BlueRidgeMark,

Unfortunately, it seems that renting them is becoming more rare these days. Perhaps some places still do, but not around here. My local places rent lots of tools, but they won't do fuel pressure gauges anymore because of the hazmat issues. On a positive note, they can be got for about $50 (or less) and are a great thing to have in the kit.

75kona

Yes it did run with holding the throttle wide open for id say around 5-10 seconds but no more than that. Last night I ordered a OBDII scanner and it should be here in a week or so. So if the Crank sensor I bought that’s coming does not solve it I will wait to get the scanner before throwing more parts at it.

Thanks for the tip ill see if my local parts stores will let me rent one. I hadn’t considered multiple problems… that would suck! Haha I will start with the part I already ordered and go from there! Thanks for the tips!

And in regards to the Crank Position sensor… I did order one off of RockAuto… are the aftermarket ones junk?

jordan96xj

That is the general consensus, yes. They can be especially frustrating when they are bad out of the box, and you put in what you think is a "known good" and turns out (after much hair pulling) to be an "unknown bad".

Even if the new aftermarket CPS works initially, there is a general consensus that they typically only have a fraction of the service life that the OEM sensor has.

75kona

Well since it came in the mail today I suppose ill install it this weekend and see the result. If it fixes it I will purchase one from the dealer to be safe. And if it doesn’t work will assume it could still be an issue and wait until my scanner arrives.

Question, are the OBDII scanners sophisticated enough to have a code for each faulty part possible? I have never in my life used one and don’t know what to expect. Thanks!

tjwalker

Interpreting any OBDII code can be tricky. I cannot tell you how many times I have seen people read a code, see a part listed in the code, throw that part at it and still have the code. And then wonder why. It usually is not straight forward and a code number doesn't tell you exactly what part to replace. Not by a long shot. Personally, I really enjoy the challenge of figuring OBD codes out and resolving them!

There are a lot of codes that have a hidden root cause. Only by understanding how all the circuits work together as a system and exploring all possible causes and using a testing based strategy, can you come to an efficient strategy of resolving codes.

Start with this strategy: 1) Post your exact code(s) here for comment, not just a description of the code. With OBD, the devil is definitely in the details. 2) Using Google, search for your exact CODE NUMBER + 1997 JEEP CHEROKEE or whatever year XJ you have. The information you will find with a simple search is invaluable in figuring codes out! Somebody else undoubtedly has had this issue and you can learn from them.

Good luck and keep us updated; we'll help if we can!

jordan96xj

It's not so much about the scanner as it is about the age of the vehicle. The scanner just shows the code(s) stored by the vehicles computer. On older OBD2 compliant vehicles (like mine, I have a 96 XJ, the first year of OBD2), less codes were implemented, and less components are tied into the OBD2 sensor management.

But let me give an example of why a code only gives you a place to start many times. Imagine a code like "POXXX - High Voltage detected in Component X". See the challenge? Its good that the computer has at least given you an idea of where to begin, but it will be up to you to figure out the possible conditions under which component X could see a high voltage reading. Usually this will take you on a journey through wiring diagrams, etc.

Or for example the engine lean/rich codes. These are really helpful for letting you know that your air/fuel ratio is having trouble. It is typically triggered when your computer can no longer compensate for the air/fuel mixture being out of tolerance. The problem is, it won't typically tell you why the air/fuel ratio is out of tolerance. (O2 sensor malfunctioning, air flow sensor malfunction, air leaks, fuel delivery problems, etc).

So typically, the codes do not pinpoint the root of the problem. Instead they try to let you know that there -is- a problem, and where to start as far as symptoms go.

75kona

Well if the one sensor doesn’t fix it I will wait for my scanner and post the codes! I would be very appreciative is you would help point me in the right direction based off of them!!

Both auto stores close to me do not have a fuel pressure tool to rent so I’ll have to track one down for this weekend’s shop time…

Thanks for your help! Feeling more confident now having help and experience when tackling this problem!

BlueRidgeMark

TJ covered it (of course!) but I'll just add this: Codes are clues, not answers.