2004 Dodge Ram 1500 5.7 Hemi 4X4, 170,000 miles. Bought with 150,000, have done regular maintenance since, including the BG throttle body cleaner, regular oil changes, and transmission fluid change.

Origination of the problem: I was driving on the highway and the truck started to have a very rough idle. If I kept it above 2500 rpm it was fine, but foot off the gas the idle is very jumpy. I managed to make my way home, keeping it in 2nd gear so RPM stayed high, around 2500 so that it would not stall.

It can start with a foot on the gas pedal, but only runs for 10 seconds, again jumpy rpm, and eventually stalls out.

I had a recently put a new fuel pump on i, and have used the truck for about a month. First thought that the pump went bad (It was aftermarket), so I went ahead and put a Dodge fuel pump on,and the same problem persisted. All connections are correct, new fuel line clips and all hoses are intact, no cracks.

I have read some similar posts about rough idle, and the various solutions ( IAC, TPS, Vacuum leak) but I want to know some steps to narrow it down. What would be the logical way to diagnose this?

To summarize, truck needs a foot on the gas pedal to start, and will stall unless immediately shifted and driven to keep rpms high. Thanks

 

Tps will mainly help for figuring out if the throttle is the issue, Iac will help for figuring out if it is an issue with idling in particular. Probably would start with the Iac, then the Tps. If I remember correctly, they are very close to each other.

 

EDIT: Sorry. For some reason I read 4.7, not 5.7

Check the compression in all cylinders. Bad compression in one or more means a valve is not closing for some reason.

Even if compression looks good, I'd pull the valve covers and look for dropped rocker arms. The 4.7l engines are notorious for dropping rocker arms and when this happens to the exhaust rockers arms, the engine runs very rough and can stall because it's pushing the exhaust gas back into the intake when the intake valve opens.

If a rocker arm did drop, most of the time you can just pop them back in and be good to go. However, sometimes, they drop because the valve seat fell out causing the valve to stay open. If this happened, it'll damage the valve, head and possibly the piston. Normally, the seat only drops when the engine has overheated. Even if it overheats for just a few seconds, this can happen. Did you have any issues with overheating?

 

The 5.7 Hemi's TPS & IAC is built into the operation of the electronic throttle body and you cannot service it, you would need a new/used throttle body if that was the cause of your problem.

I would start by plugging in an OBD-2 scanner and see if you find any codes. Then if it has live data you will want to see what your LONG TERM FUEL TRIM readings are for each engine bank...make sure you ONLY pay attention to the long term readings for Bank 1 sensor 1 and bank 2 sensor 1 readings. If any value is exceeded by +/- 10 that bank is the side with the problem, a +10 reading would indicate that o2 sensor is telling the pcm to dump more fuel into that bank to compensate for something like a vacuum leak, to confirm vacuum leak, give a slight rev and the number should even out. When reading fuel trims you always want to see numbers close to 0, but anything below 10 +/- is absolutely within the PCM's ability to compensate.

For example if LTFTB1S1 (long term fuel trim bank 1 sensor 1, sometimes read as LONGFT1 depending on the scanner) If it reads anything above a +10 like a +14.3 for example that means the o2 sensors are telling the pcm it needs to add fuel to compensate for incoming air, this can also indicate a fuel injector problem if when you rev the engine the long term trim doesn't dip back toward zero but rather goes even higher than the 14.3 example number then you have a fuel delivery issue. Its a bit complicated but can be a really great diagnostics trick for your diagnostics tool bag. A number that is in the -10+ range means the engine has too much fuel for the air coming into the engine, would indicate something like a clogged air filter or stuck fuel injector. If the fuel trims state a problem on one side of the engine then its side specific, if it states an abnormal number on both bank 1 and bank 2 sensors then its a problem with a component that feeds both sides of the engine like fuel pump, air filter, throttle body ect. Theres plenty of videos on youtube explaining how to use fuel trims for your diagnostics, but this is the severely condensed version.

If reading fuel trims seems tricky, start by narrowing down the problems this way....

Go to Harbor Freight and buy a Vacuum gauge ($10)and a Fuel Pressure Gauge ($19). Fuel Pressure should be around 58-61 psi and be steady, though honestly anything above 50psi is acceptable. Many Holley Fast EFI setups only require about 45psi on their near stock 5.7 Hemi programs. Steady vacuum readings of 15-20 inches are normal, you do not want to see the needle bouncing or fluttering. Connect to ports on intake behind throttle body on drivers side...follow brake booster hose to where you need to hook gauge and disconnect brake booster hose or the other smaller vacuum hose to tap your gauge into, do NOT disconnect both vacuum ports at the intake during the test!, only one at a time and make sure the one disconnected goes into your gauge.

Start there and see what you find!

 

Did you ever figure out what the issue was? Having the same issue with my truck :/