1988 Jeep Cherokee

1983-88 Exhaust Emission Systems JEEP EXHAUST GAS RECIRCULATION



Valve Opening Test

  • With engine at normal operating temperature and at idle,
    rapidly open and close throttle. Open throttle sufficiently to obtain
    at least 1500 RPM. Movement should be noticed in EGR diaphragm.

  • If diaphragm does not move, probable causes are: faulty
    vacuum signal to EGR, defective EGR diaphragm or defective
    backpressure sensor diaphragm (if equipped), or leaks in vacuum lines
    or connections.

Valve Closing Test

  • With engine at normal operating temperature and at idle,
    manually depress EGR valve diaphragm. RPM should immediately drop,
    indicating that EGR valve is not leaking and had been properly
    cutting off exhaust gas flow at idle.

  • If there is no change in RPM and engine is idling
    properly, exhaust gases are not reaching combustion chamber. Check
    for plugged passage between EGR valve and intake manifold.

  • If engine idles poorly and RPM is not greatly affected by
    manually moving diaphragm up, EGR valve is not closing off exhaust
    gas flow. Check for carbon between pintle, leaking EGR valve gasket
    or bad EGR valve.


NOTE: Engine coolant temperature must be below 100F (38C) to perform this test.

1) Check vacuum lines for leaks and correct routing.

Disconnect vacuum line at backpressure sensor (if equipped) or at EGR valve, and attach this line to vacuum gauge.

2) Operate engine at 1500 RPM. No vacuum should be indicated
on gauge. If vacuum is shown, replace CTO switch.

3) Idle engine until coolant temperature exceeds 100 F
(38C) on 4-cylinder engines, or 115F (46C) on 6-cylinder and V8


4) Raise engine speed to 1500 RPM. Ported vacuum should be
shown on gauge. If not, replace CTO switch.


  • With engine at normal operating temperature, remove dump
    valve vacuum hose from manifold and plug manifold connection.

  • Raise engine speed to 2000 RPM. Vacuum should be present
    at exhaust ports on bottom of valve. If not, replace valve.

  • Reconnect vacuum hose to manifold and raise engine speed
    to 2000 RPM. No vacuum should be felt at exhaust ports on bottom of
    valve. If vacuum is present, replace valve.


1) With the air cleaner temperature below 40F (-4C), disconnect vacuum hoses from TVS and connect vacuum source to large


  • Apply vacuum to TVS. TVS should hold vacuum. If not,
    replace TVS.

  • Start engine and warm air cleaner to 55F (13C), or
    greater. TVS should not hold vacuum. If it does, replace TVS.