1999 Jeep Grand Cherokee 4.7L V-8 Idle Speed Issue

[crazy for the 4.7]

Hello everyone,  I have been troubleshooting various electrical issues for months. I am fixing the issues as the occur, however one issue I cannot seem to determine the cause. A 1999 Grand Cherokee WJ with an intermittent low idle complaint.  The idle drops to 300 rpm in drive only. I currently have no trouble codes. I have checked all sensor voltages and they are all correct. I did change the TPS and the IAC anyway, but the issue is still occurring. Not sure where to go from this point, so I really do appreciate any help or suggestions you can provided.

[Moses Ludel]

Welcome to the forums, "crazy for the 4.7".  You were on the right track with the TPS and IAC, I would have looked there, too.  Are you throwing a code?  This could be transmission related from the transmission module, which would be a DRB-III or higher end scanner read.  Check the plug connectors that deliver transmission signals.

I would look at the MAP sensor, too, just to be sure.  You share that all voltage sensors check okay, look at the vacuum circuits as well, especially to MAP.  Again, try to get any stored codes.  

The 4.7L V-8 is impressive if folks would not overheat it and complain about bad cylinder heads and valve seats.  Your comments and experience with the 4.7L V-8 engine would be helpful.

Moses

 

[crazy for the 4.7]

Thank you Moses for the quick response to my concerns. I have an appointment in the morning with a local transmission shop to check for transmission codes via the TCM and I will post the results tomorrow. I would be glad to assist any members who may have any issues which pertain to the 4.7l V8 or the WJ's in general. For reference the transmission has no issues and the motor runs really strong despite the high mileage. Therefore the cause has not been obvious.

[Moses Ludel]

Understand the difficulty with some troubles.  Interested in your findings from the TCM codes.  

It's hard to get DRB-III level troubleshooting results without that scan tool, and the last round for the DRB-III from OTC/Miller/SPX was a rental program with a $6K deposit.  The tool is valuable but scarce, out of production for years now, and the alternative is an aftermarket scanner like your local transmission shop uses.

I would guess that many Chrysler dealerships are no longer supporting DRB-III level troubleshooting, especially if their older DRB-III scan tool has failed.  I have the newer StarScan® tool that only works for a window of vehicles, it was touted as the wave of the future when launched.  The architecture is similar to the DRB-III but there was never software developed for the StarScan® to be backward compatible.

I really like the '99-'2004 WJ Grand Cherokee!  It's the last 'Grand with the beam front axle, still boasts the 4.0L base engine, and of course, the higher tech 4.7L V-8 was a boon after the ZJ with its dated LA 5.2L and 5.9L pushrod engines.  The 4.7L gets a bad rap for valve seat failure and "dropping seats", though my sources in the reman and machine shop field say this is typically the result of overheating the engine.

What's your experience with the 4.7L and valve seats? Head issues in general?  And how about the HVAC system control switch that is failure prone and very expensive?  Have you dealt with any of these issues?  A new topic on these subjects would be helpful to others!

Moses

 

[crazy for the 4.7]

Hello fellow jeepers. Update, I had a local transmission shop (one I trust) just inspect the transmission mostly to eliminate the transmission as a cause to my issue. Also due to the high mileage an eval on a fluid change with possible upgrades possibilities (reprogram kit, solenoid etc...). The transmission is great no worries and no TCM codes found by the transmission shops scanner. Therefore safe to say the transmission is not causing my low idle problem.  Background. This issue started when I replaced my catalytic converter and also upgraded to a single three inch exhaust. The truck already had prior, an after market Y pipe and a three inch cat converter, but also had a cat back borla (2.5") exhaust. The catalytic converter was bad so I replaced it with the same converter and upgraded to the larger exhaust pipe (cat back). The 4.7l v8 works really well with this setup (single pipe only).  Soon after doing this is when the issue began. Eliminating the transmission I began to check the map sensor as Moses suggested. What I found was the engine is making 19lbs of vacuum at idle. However the MAP sensor voltage was all over the place and at intermittent times only reading 4lbs of vacuum. Also the 1/1 (upstream) O2 sensor was reading a steady voltage at idle. While driving the truck, the O2 sensor was working properly (I don't have the voltage readings) but it did read high and low as it should. At idle it would read the same voltage and did not change. I will check the voltage to the Map sensor to eliminate a wiring issue. And since the O2 and MAP sensor are related I may change the O2 sensor as well.  Additionally I don't think at this point the truck is loosing vacuum, so I don't think the cause is vacuum related.

[crazy for the 4.7]

I meant to respond to you Moses in my previous comments but was distracted. I have had ALL of the issues with my 99 Grand (WJ) that all others have had throughout the years. I have also read through countless forums in search of solutions and what I find a lot is everyone has had the same questions but cannot find true and accurate resolutions. 

As far as the heads, so far no issues. Cam changes are not extremely difficult, and upgrades are available. A good machine shop can address and fix the valve guide issue. The only issue I have with the 4.7L are the ring lands are to tall (close to the top of the pistons). Poor set up for performance especially boosted applications. And also not much in after market choices for the Jeeps (more so for the Dakota and Durango market). 

The 4.7L does not like heat as stated and cooling is a priority. The electric fan is huge and a constant issue. The  tow package includes the mechanical clutch fan and the electrical fan (possible upgrade option). Also aftermarket transmission coolers anything to keep the temperatures down Always get Chrysler fans and relays. This is a huge job to replace them and not one you want to do twice if you can help it.

Also I recommend upgrading to a larger aluminum radiator (two or three core). Helps a ton especially in hotter climates with the A/C on continuously. The truck runs five to ten degrees cooler since doing this.  As for the HVAC my dash as been out three times since new. Mostly for the heater core and the blend doors changes. I would change the Evaporator also if you have reached this point of repair. The HVAC control switch is still the factory switch and has not been a problem as of yet. Although I do not have the dual climate control switch I believe you are referencing.  I have found the parts yard is a good source for replacing them cheaply if need be. Other than that the system has worked as advertised so far.

The WJ is a great truck and the 4.7L V8 does not get the praise it deserves and unfortunately there is not a lot of aftermarket for them (for the motor heads out there). I would be happy to assist anyone in answering or discussing any topics or issues relating to the Jeep Grand Cherokees.

Thank you all in advance for the opportunity to address my concerns as well, this a great forum with a lot of experiences being shared.  

[Moses Ludel]

crazy for the 4.7L...Thanks much for sharing your experience and making a thoughtful, objective assessment of the WJ Grand Cherokee's 4.7L V-8.  I am a real fan of these models, initially applauding the continued use of the beam front axle.  My friend Jim Frens was VP of Jeep Platform Engineering when the WJ was on the drawing board.  He fought for the beam front axle and won.  He since moved on, and Chrysler did, too.  

Though the Wrangler has persisted in using beam axles front and rear (largely for marketing reasons and not necessarily for the beam axle's proven reliability/simplicity or the sound logic of truck-grade beam axle stamina), the 2005-up Grand Cherokee moved into the realm of light-duty SUVs and crossover cars.  Looks like a Jeep right down to the grille marque and badges yet functions like a car!  Late Grand Cherokee service chores are much like a contemporary car, too.  A 2005-up Commander, Grand Cherokee, Patriot or the contemporary Cherokee is not in any way "fun" to work on.  Realistically, the 2012-up Jeep JK Wrangler takes a full 8-hour work shift to change out the heater core.

Your input on the Jeep WJ is valued and applauded.  I'm trusting that others will respond with questions and new topics that you can address from your experience. 

Thanks!

Moses

[Moses Ludel]

I like your approach to the MAP and O2 sensor.  Regarding replacement parts, the only O2 sensor I use is Mopar-packaged or the OE supplier.  On my 4.0L XJ Cherokee, that supplier was NTK, and I could match the OE sensor's part number to an NTK item.

You commented on your experience with engine cooling fans.  There are some aftermarket pieces that work, but electrics, and especially sensors, seem best when OE sourced.

As a footnote, I've had issues with an O2 sensor, CTS switch and a TPS switch from offshore aftermarket brand sources (purchased at well known parts houses).  These parts either do not work to spec and calibration or have a high failure rate. Most of these issues are due to the "universalizing" of the parts for wider distribution. 

Moses

[crazy for the 4.7]

Well I replaced the Map sensor today and although the jeep is running better, the condition still exist. An area of improvement is the transmission is shifting much firmer in all rpm ranges. I did not change the oxygen sensor yet (has not come in yet). My hope is that the o2 sensor is the fix, we'll see. Moses I agree with you in that I also only purchase Chrysler /jeep  products especially when it comes to the sensors. Its worth spending a little more for the confidence that the issue will not soon return. Moses I do want to ask you, I know the TPS sensor is for the most part plug and play. However can the TPS be fine tuned via the voltage to find the engines peak performance (sweet spot). I have done this in the past with other brands but you need to know what the voltage numbers are. Are you aware of tuning the jeeps in this way.

[Moses Ludel]

crazy for the 4.7...The G.M. TPS switches were adjustable.  They had slotted TPS screw attachment holes.  Adjusting the voltage settings at idle were a regular part of installing a new TPS, and older switches could often be simply readjusted.  Sometimes, the TPS voltage is tested and adjusted at various degrees of throttle valve opening.

Mopar and others, as you know, have eliminated the slotted TPS screw holes.  (Likely there is a press to do this to meet emissions compliance and standards.)  With the Jeep engines (and others), you install the new switch, and if it reads within the OEM voltage guidelines at an idle position or a given throttle valve position, that's good enough. It's good enough because the PCM reads a variety of sensors, in particular the upstream O2 sensor and MAP, and then controls the fuel/spark parameters. In order to process sensor feedback, the PCM needs only a baseline or base input signal from the TPS.  

Technically, you could move the TPS to change calibration by grinding the slots slightly in each direction if you could avoid damage to the internal components of the switch.  Would I do this?  No.  It would be more sensible to use the properly calibrated TPS switch (meeting the OEM voltage parameters) and apply aftermarket software to "tune" performance into the engine.  By moving the TPS, you would be changing the voltage reading across the entire rpm and throttle valve angle/range of the engine.  There may be a sweet spot at one point that becomes a not so sweet tuning modification elsewhere...This would be like taking a fixed curve mechanical/centrifugal advance ignition distributor and advancing the base timing for more bottom end performance:  The spark timing at the mid- to top-end rpm of the engine could become too advanced, creating spark knock or detonation.

So, I'd stick with the OE settings for the TPS, using the normal voltage reading to the PCM as a baseline like we do with the CPS (crankshaft position sensor).  You want CPS to indicate a true TDC of #1 piston.  The software program in the PCM determines how best to tune the engine for various loads, atmosphere, temperatures, air/fuel ratios, (sometimes spark knock) and so forth.  TDC is simply a baseline parameter.  The same can be done with the TPS voltage readings.  All that TPS really does is tell the PCM what the throttle valve angle reads—taking the driver's mechanical input at the gas pedal and turning it into a voltage reading!

There are aftermarket tuning programs that build from scratch.  Given the potentially infinite and overlapping timing and fuel algorithms, it is complicated to build fuel and spark management curves and programs.  Performance tuning modern electronic fuel-and-spark management systems is a science, much of which begins with baseline or OEM software programming.

Early in the EFI era, I wrote for OFF-ROAD, Popular Hot Rodding and several other performance magazines.  I played with an Electromotive fuel management computer and G.M. TBI unit.  The system required from-scratch software creation then a PC or laptop upload.  The engine, a relatively stock 4.2L Jeep inline six, had a conventional standalone distributor and merely required fuel curves to match the fixed spark curves...This would be similar to a Howell Engineering TBI conversion for a Jeep 4.2L that also uses a G.M. TBI unit but still keeps the Jeep/Motorcraft 4.2L conventional distributor or uses a similar aftermarket distributor with fixed centrifugal and vacuum advance mechanisms.  

The Electromotive experience, done properly, was not simple and proved humbling.  Over-fueling the engine could fuel-wash the cylinders and wipe out rings and valves.  Too lean at the wrong time caused detonation (ping or knock).  At the OE manufacturers' level, there are entire departments dedicated to building fuel-and-spark tuning and emission software.  Fiddling with the TPS voltage could have significant trickle-down effects...

Moses

[crazy for the 4.7]

Hello Moses. Replacing the MAP sensor and the bank 1/1 O2 sensor did not correct my problem. Bank 2 O2 has been replaced previously. I did another live scan and have found the fuel trim is  possibly in a rich condition. The long term fuel trim is negative (-7.2 to -12.5) and the short term fuel is 0 to -3. I think I narrowed it to bank 1 drivers side, but not completely sure. Now the fuel filter and the k&n air filter are old so I will change them , however unless its a PCM issue, I'm leaning towards a leaking injector. Can I test for a faulty injector without removing them from the truck. The motor runs really well other than this idle issue, so I'm not thinking coil pack, but I would like your opinion. Also how different is the PCM tables in drive with a load verses in park. The issue only occurs in forward gears not in park (idles down from 650 RPM to 450 Rpm). 

[Moses Ludel]

Here are some thoughts about your current assessment:

On 1/20/2017 at 1:58 AM, crazy for the 4.7 said:

Now the fuel filter and the k&n air filter are old so I will change them , however unless its a PCM issue, I'm leaning towards a leaking injector. Can I test for a faulty injector without removing them from the truck.

The PCM would be a less likely source for this problem, as the PCM either works or doesn't, it faults consistently and fails to deliver.  (PCMs do fail, so you're not amiss thinking in these terms, but the PCM would not be at the top of my list.)  You can test the injector pulse with a surface contact tester, simple to do, but this will not pinpoint "leaking", rather it would tell whether the injector is pulsing.  If it's definitely not an O2 sensor issue, the injector(s) could be faulty.  

Here's the tool I have and use for quick and reasonably accurate testing of the electric-pulse pintle movement within the injector.  However, the pintle solenoids switching on and off are not a means for sensing the actual volume of fuel flowing through the injectors; this would only tell whether the PCM is firing the injectors and whether the solenoids work or not. I hate to spend your money, but this tool is very cool and a great toolbox companion for trail running.  Makes you popular around the campground:

https://www.amazon.com/Waekon-76462-Universal-Electronic-Injector/dp/B0021V0FRE 

The motor runs really well other than this idle issue, so I'm not thinking coil pack, but I would like your opinion. Also how different is the PCM tables in drive with a load verses in park. The issue only occurs in forward gears not in park (idles down from 650 RPM to 450 Rpm).

If we shift away from a fuel issue and lean misfire, of course there's the ignition, and coil-on-plug can be a PITA.  Again, Waekon has a quick tester for the coil-on-plug units, and its cheaper to buy the diagnostic tool than to arbitrarily replace the coil packs and spark plugs on a hunch:  https://www.amazon.com/Waekon-76562-Quick-Variable-Sensitivity/dp/B0021UPE58/ref=pd_bxgy_263_2?_encoding=UTF8&psc=1&refRID=F87RJMRMBTSEYSPJB0Y3.  I have both Waekon tools and feel fortunate, they provide quick tests that can save a great deal of time and second guessing.  Symptomatically, you do have either a misfire or a cause for lean burn:  The question is why. 

Low speed idle but at a constant speed still takes me back to those signals related to the engine under load.  My thought is that there is no signal indicating the transmission is in gear and the engine is under load.  The IAC should be compensating and raising the engine speed, and it either can't do its job or it is not getting the right signals.

I still have issues with the engine dropping speed uniformly.  Fuel starvation and spark misfire usually cause an intermittent rise and drop in engine speed.  You've ruled out MAP.  Could you be bucking a simple vacuum leak or one-way vacuum check valve issue?  I had a minor vacuum hose failure alongside the battery on our '99 4.0L XJ, where the reservoir stores vacuum.  Battery acid ate the hose, the symptom was the cruise control not holding on a grade when manifold vacuum dropped.  The A/C would also stop working for the same reason, a weak vacuum signal to the A/C controls.  

I would check your manifold vacuum when the engine is dropping the idle speed.  Keep the engine running so the vacuum reading is somewhat accurate.  Then check various vacuum points underhood where you should be reading manifold vacuum.  If necessary, verify whether the one-way vacuum check valves work, they're inexpensive and could be a source of trouble.  Look for vacuum leaks in general.

We talked about heater and A/C issues on the WJs, consider any vacuum hoses, circuits or vacuum switches that could cause a vacuum leak.  Fix these leaks and see if that answers some questions...Also consider vacuum at the EVAP system and the EVAP function as a whole.  Don't rule out the simple issue of a defective gas cap.

While you are troubleshooting vacuum, do not overlook the vacuum hoses and also the function of the Leak Detection Pump (LDP).  These devices are hidden, they have a paper pleated filter, and the LDP is notoriously disregarded on Jeep WJ models due to lack of awareness and terribly difficult access.  The pump itself can leak, the vacuum hoses and elbow can leak, and the LDP is clearly a prospect for trouble.  (One hose actually goes directly to the intake manifold.)  A caution here:  Do not attempt to blow out the LDP or EVAP hoses with compressed air!  This very low pressure system will not tolerate high pressure; you do need to clean things up, but not with your air compressor...