4.0L Stuck In Open Loop - Almost Stalls In Closed Loop

[Ωhm]

Without having a fuel pressure gauge to determine if fuel psi is changing when vacuum is removed from the fuel pressure regulator at idle, try using MT2500. When engine is at idle and in closed loop remove vacuum source at FPR and see if INJ PW time decreases or/both if STFT starts to lower in numbers.

 

Also closed throttle position is learned. Meaning you may have to give it some time to learn.

[cruiser54]

If you're getting 39 PSI of fuel pressure at idle instead of 31, you will have a low idle.

 

TPS should be 17 or less. Sort the fuel pressure stuff next. 

 

And, Renix doesn't learn TPS adjustment. The ECM sees exactly what the MT 2500 does. 

[boxyjeep]

I've got the TPS set to 17% throttle at idle and 0.84-0.86v.  Idle is still super low and unstable.  Will order a fuel pressure tester and see how that looks.

 

Also got some new plugs since they looked a little fouled.

[cruiser54]

17 hours ago, Chargum85 said:

I've got the TPS set to 17% throttle at idle and 0.84-0.86v.  Idle is still super low and unstable.  Will order a fuel pressure tester and see how that looks.

 

Also got some new plugs since they looked a little fouled.

Excellent plan.

[boxyjeep]

Ok, got the fuel pressure gauge connected.  Reads about 30-31psi at idle.  Disconnecting the FPR it spikes up to 39-40psi.  When giving the engine throttle, it stays around 30psi and fluctuates up or down 2psi.

 

Should the fuel pressure be increasing with throttle or maintaining close to 30psi?

 

Also changed the plugs today.  No noticeable improvement.

[boxyjeep]

I've been reviewing this a bit more and am considering a possible clogged cat or bad fuel injectors.  Will also be targeting to eliminate the C101 connector this weekend.  The truck was in rough shape when I got it and despite this rough idle, it is running the best it ever has.  When I got it, the block-side coolant temperature sensor was unplugged (and wiring cut), the fuel pump ballast resistor was bypassed, the O2 sensor was forcing a rich condition, and it had a bad exhaust manifold/gasket (among many other things).  I'll also target to disconnect the exhaust before the cat to see if that shows any noticeable improvement before I buy any new parts.

[mjeff87]

I was going to suggest a bad O2 sensor before even reading the whole thread....glad you got that squared up

 

On the low idle speed, go old school first.  Pull and inspect/clean the IAC first to see if it makes an improvement.  With as much hackery going on to your rig as you say, it very well could be all bunked up.  Start simple, then work your way up from there.

[boxyjeep]

40 minutes ago, mjeff87 said:

I was going to suggest a bad O2 sensor before even reading the whole thread....glad you got that squared up

 

On the low idle speed, go old school first.  Pull and inspect/clean the IAC first to see if it makes an improvement.  With as much hackery going on to your rig as you say, it very well could be all bunked up.  Start simple, then work your way up from there.

 

Sorry, forgot to mention.  It has a new IAC (same behavior as the old one) and cleaned throttle body/new TB gasket.  I've replaced almost every vacuum line that even looked close to failing and done a handful of vacuum leakage process-of-elimination tests by disconnecting and capping off certain ports.  Unfortunately, still the same.  I'm really leaning towards dirty/bad injectors or clogged cat.  Both appear original on this 30-year old 211k truck.  It was at a Southern CA farm most of it's life in very hot, dry, dusty conditions.  Luckily no rust, but most of the plastic is melted/brittle.  Also, it accelerates somewhat well, but still feels like something is restricting at higher RPM's.  I'm comparing this to my seat-of-the-pants feelings from older XJ's that I had 10+ years ago, so this level of performance might be the norm.

 

It also has a new distributor and plugs.  I haven't indexed the distributor by cutting off the ears yet, but don't believe that would cause such a big issue at idle.  I'm also somewhat concerned with timing chain slop, but it didn't seem too far off when rotating the crankshaft and watching the distributor rotor move simultaneously.

[cruiser54]

You can test for a clogged cat.

 

CRUISER'S MOSTLY RENIX TIPS

VACUUM TEST FOR EXHAUST RESTRICTION

OCTOBER 31, 2015 SALAD 1 COMMENT

 

Your vacuum gauge should come with an instruction booklet outlining the procedure.

Hook the  gauge up to a vacuum source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum.

Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle.

Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.

[boxyjeep]

Thanks--that's helpful.  I'll get a vacuum gauge and test it.

[sporttruck88]

I had the same trouble with my '88, after I swapped my '90 4.0 in; when I changed the EGR valve and IAC valve it started running right. 

[cruiser54]

3 minutes ago, sporttruck88 said:

I had the same trouble with my '88, after I swapped my '90 4.0 in; when I changed the EGR valve and IAC valve it started running right. 

 

 

EGR could be leaking internally allowing exhaust gasses to go into the intake manifold at idle.....