Hey guys, new to the dakota world. I recently sold my Jeep cherokee and found a mintyy 1989 3.9 TBI dakota sport with only 111k miles, 5 speed manual, 4x4, factory light bar, no rust, and aside from some clear coat on the roof and hood and the AC not working everything is almost perfect.

Which brings me to my trauma from the last couple weeks... the dakota has this intermittent issue where when I start it up it'll start fine, sometimes when it's cold it'll take a second or two but then fires up. About half the time after it's just started up, it will bog down if I give it too sharp throttle (feels like a bad accelerator pump in a carb almost) and sometimes when driving in gear at steady throttle it'll buck forwards and back like im stabbing the throttle and letting of over and over again. usually once it's up to operating temp it runs perfect though.

Another potentially related issue which is more of the annoyance here, it will randomly not start when it's warm/after running. I'll start it up, let it run/drive it/idle it and then if I shut it off and try to restart it it will crank but not start. I've experimented a bit, and sometimes I can get it to start back up by unplugging the engine speed motor/throttle kicker thing and then trying, but usually just waiting 5-10 mins does the trick. I also tried spraying carb cleaner down the throat when it wouldn't start and that also did the trick every time I tried it.

I just replaced the fuel pump since I thought it could be the issue, and because it was the original fuel pump and I wanted to get a little ahead on maintenance, and it made no change aside from it starting up a bit quicker/smoother now. I also replaced the temp sensor since I read that could be a potential issue but that also made no difference.

The engine is showing no codes, just does the 5-5 flashes when I cycle the key. I know these TBI's can be a bit finnicky so I wanted to get some more knowledgeable people involved haha. I couldn't find any vac leaks with a can of spray cleaner, but that is also possible I suppose. I am assuming the culprit may be the the TPS acting funky when it's warm, or maybe even the fuel pressure regulator. I am kinda lost on this and am trying to keep myself from just splurging and the auto parts store and replacing every sensor until it works right haha. Thanks in advance!

 

Do you have a scanner that can read live data? Wonder if the TPS is giving some bad readings until it warms up.....

 

I do not, I’ve been leaning towards tps as well though. I may just get one and swap it real quick since they’re only like $30 or something. The biggest question is if that’s the cause of the intermittent starting issue I’m having. I think it’s either that or the fuel pressure regulator since the idle air control/engine speed motor thing is working fine and it’s not throwing any codes so I don’t think it’s the map or 02 sensor

 

O2 sensor wouldn't be involved in starting. Wouldn't hurt to check the vacuum line between the throttle body and the map though.... if there is one.... check the o-ring if there isn't a line.

 

alright, I caved and replaced the TPS and replaced the driver side injector as it looked like the spray pattern was a little off when I examined it running the other day. That all being said, it made virtually no difference lol. When it's warmed up I definitely feel like it runs a bit smoother and revs more freely but the warm start issue and the bogging issue still persist.

I checked the vac line going to the MAP sensor and it looked fine, I just rechecked for vacuum leaks with a can of starting fluid and made sure to spray by the map and there was no leak there or anywhere else on the engine that I could find.

I did however try unplugging the electrical connector on the MAP while the engine was running and it didn't really seem like it made a difference... I heard a slight stutter and then it still bogged down when I tried to rev it and otherwise seemed like it was running the same barring the check engine light coming on. I plugged it back in and again it didn't really make a difference in how it was running.

At this point im definitely going to chalk the TPS, coolant sensor, and fuel pump as preventative maintenance but I'm leaning towards the culprit being either the fuel pressure regulator since I know those diaphragms can be finnicky with temp changes when they get old, or of course the MAP since it didn't seem like it was doing much other than keeping the check engine light off. thoughts? I also read another post from 100 years ago where someone was having starting and running issues and it turned out to be the EGR so I'm wondering if that could be something to explore as well

 

EGR is definitely a possibility...... Usually though, EGR will set a code.. But, not always.

 

I know what an EGR valve does but I don’t know how it does it. If an EGR valve goes bad what typically fails with it?

 

They have some interesting failure modes..... stuck closed, or clogged passages, you may get spark knock, or similar, stuck open, lack of power, running rich, black smoke, etc. And a host of other fun symptoms.....

 

Kinda sounds like a $50 preventative maintenance part with all that going on.

 

Swap your fuel pump relay with a known good one. It might not help but it doesn't cost anything. If you have A/C, it uses the same relay. A relay that fails when it's hot can be a stinker to track down. It can have you banging your head. I can think of other thing, but try the cheap and easy first. If it works, great. If not, no money and very little time lost.